Here's a list of equipment which I've selected specifically for you base on your requirements, namely:

These are 23" monitors which use IPS technology. While they are more expensive than  ordinary (TN) monitors, they offer much better colour, and within their price bracket these are considered to be the best "graphics" screens. Natrurally, you get what you pay for, so the ViewSonic may not be every bit as good ass the Dell or the NEC, but reviews on the internet are very positive about it too.

Since good colour rendition is important to you, I left some slack in the budget for a colour calibrator. These are not so expensive nowadays, and can make a huge difference to the performance of the monitor.

One which I have seen recommended in a number of places (and indeed have used an earlier version of) is the Eye One Display 2, which you can buy for £128.70 from Amazon (http://www.amazon.co.uk/X-Rite-EODIS2-Eye-One-Display/dp/B000JLO31M/ref=sr_1_1?ie=UTF8&qid=1309274691&sr=8-1). 

Or, you can see a list of other models on the Jigsaw website.

This costs £55.37 from  Adobe's UK shop .

The base unit is only guaranteed for one year, but you can buy an extended warranty for it separately:

Acer Warranty - 3yr Collect and Return for Acer Aspire Desktops and TFT Models £71.99 inc. vat
http://www.ebuyer.com/product/198709

Reference no. 1423284N
Acer Repair Centre
Unit M Eagle Road
Plymouth PL7 5JY