As predicted, Apple has updated its entire range of iMacs to bring the Retina screen to the smaller models for the first time and give the bigger models new Skylake chips. Here, we bring you everything you need to know about the new 2015 iMac line-up, including their release dates, price and specs as unveiled by Apple on 13 October.

It's been over two years since Apple last updated its four original non-Retina versions of the iMac (if you don't include the cheaper iMac which Apple introduced in June 2014 - now more than a year old). The big surprise was that Apple left the 21.5in iMacs (which previously lacked a Retina display option) completely untouched despite recently adding a new, cheaper 27in Retina model to its line-up.

We had originally expected new iMacs to be shown off during WWDC 2015 back in June, and then during Apple's 9 September event, but the company has instead chosen 13 October to quietly launch the updates.

"From the first iMac to today, the spirit of iMac has never wavered – deliver the ultimate desktop experience with the latest technologies, gorgeous displays and cutting-edge designs," said Apple's SVP of Worldwide Marketing Philip Schiller in Apple's press release. "These are the most stunning iMacs we've ever made. With our gorgeous new displays, more powerful processors and graphics and all-new Magic accessories, the new iMac continues to redefine the ultimate desktop experience."

As ever, Phil's words are a bit on the dramatic side there, but there have been some significant improvements and changes to the iMac line-up so read on to find out more.

To discover what we think about the original Retina 5K iMac, take a look at our Retina iMac review. If you are in the market for a new Mac, read our iMac or Mac mini - Mac desktops compared and Best Mac to buy: Mac Buying Guide.

2015 iMac release date: When are the new iMacs coming out?

Apple's new 27in iMac with Retina 5K comes in three models, each available to buy today from Apple's website, retail stores and Apple Authorised Resellers, as are the two new models of non-Retina 21.5in iMac and the all-new 21.5in iMac with Retina 4K display.

Each new iMac will ship with Apple's new Magic Keyboard and Magic Mouse 2 (which we'll talk about later in this article) and there's the option to order the new Magic Trackpad 2 at checkout.

2015 iMac price UK: How much do the new iMacs cost?

The 2015 models of the 21.5in iMac are priced as follows:

1.6GHz processor, 1TB Storage: £899
2.8GHz processor, 1TB Storage: £1,049
Retina 4K display, 3.1GHz processor, 1TB Storage: £1,199.

The 2015 models of the 27in iMac (all of which come with Retina 5K display) are priced as follows:

3.2GHz pricessor, 1TB storage: £1,449
3.2GHz processor, 1TB storage (Fusion Drive): £1,599
3.3GHz processor, 2TB storage (Fusion Drive): £1,849

The great news is that prices haven't changed. Even the new 4K 21in iMac costs the same as the equivalent model did previously.

2015 iMac specs: What new features and tech do the new iMacs have?

Apple's 21.5 iMac is now available with a Retina 4K display for the first time, offering a resolution of 4096 x 2304, which equates to a total of 9.4 million pixels on the top of the range model. That's more than four times the number of pixels in the standard 21.5in iMac models, of which two remain.

All three of Apple's new 27in iMac models now have a Retina 5K display, which boasts 14.7 million pixels. Previously only two of the 27in iMacs offered a Retina display. Plus, Apple says that it's improved the colour gamut in its Retina displays too, for 25 percent larger colour space. Apparently, these displays are capable of displaying 99 percent of the P3 colour space.

Aside from the new Retina displays, all of the new iMac models look the same as their predecessors, with the now-familiar design that's impressively thin at the edges.

Beneath those Retina displays, the new 27in iMacs have Intel's sixth-generation Core processors, otherwise known as Skylake. That's paired with AMD Radeon R9 graphics with 2GB video memory.

Apple has opted to update its 21.5in iMac models from fourth-generation Haswell processors to fifth-generation Broadwell rather than sixth-generation Skylake, but Apple says this is because Intel hasn't yet made Skylake available with integrated graphics processors. The Broadwell processors in the new 21.5in iMacs sit alongside enhanced Intel Iris Pro Graphics.

The base configuration for the new 21.5in iMacs is a 1TB 5,400 hard drive, which seems a shame because you'll find it's much slower than the Fusion Drive option available if you cough up some extra cash (something we'd strongly recommend doing in this case).

One final thing, it had been so long since the 21in iMacs had been updated that they still used Thunderbolt 1 ports. Now they have been updated to Thunderbolt 2.

Read more about the specs of the new iMacs in our review of the new 21.5in 4K iMac.

2015 iMac specs: How to get 64GB RAM for the new 27in iMac

According to Apple’s build to order page for purchasing the new 27in iMac it is possible to upgrade the new models to take 32GB RAM, in four 8GB slots. 32GB of RAM costs £480 as a build to order option from Apple.

However, it has emerged that it will be possible to update the new 27in iMacs to take 64GB RAM, in four 16GB slots, just as soon as the compatible memory becomes available, according to 9to5Mac.

The good news is that where the RAM in the 21in iMacs is soldered on, the RAM in the 27in iMacs is user upgradable plug-in RAM, so, after purchase users will be able to purchase the upgrade kits from the likes of OWC, and, we expect, Crucial. OWC has announced that it will be selling a 64GB RAM kit for $1195 and a 48GB RAM kit for $729. German reseller Memphis has been in touch to confirm that they will be selling 64GB RAM for the new iMac on Amazon UK - you can buy the 64GB RAM-Kit here for £575.

New Magic accessories: Magic Keyboard, Magic Mouse 2 and Magic Trackpad 2

In addition to the launch of new iMacs, Apple has also introduced new wireless accessories including the all-new Magic Keyboard, Magic Mouse 2 and Magic Trackpad 2. Apple says they've each been developed to feel more comfortable than every, and they also feature rechargeable batteries for the first time. Plus, the Magic Trackpad 2 boasts Force Touch technology like that found in the trackpads of some of Apple's new MacBooks.

The new Magic Mouse 2 is lighter and sturdier accodring to Apple, ans should have a smoother glide than the original, while the Magic Keyboard is 13 percent smaller than its predecessor with an even lower profile. It uses a new scissor mechanism for the keys too.

More to follow (we're updating this article right this moment).

On page two is everything we wrote before today's iMac announcement. Read on to find out how much of the speculation was accurate.