I cannot be without my computer for very long, so I will be complaining to Apple re. not fit for purpose, how should I approach this?
The way I would do it is -
1) contact the store and ask to speak to a senior manager - I would then tell them that you are not happy that a product you bought from them, which is x time old and cost y is obviously not "fit for purpose" and that it is 'it is not of satisfactory quality' and as your rights under the "Sale of Goods Act 1979 (as amended)" you want them to
a) either repair the imac at no cost to you (i would also suggest that they should give you a "loan" machine if it will take sometime to repair) or
b) Give you a refund*
* point about refunds is that they have the right to deduct a % of the price of the goods according to how long you have had it - so I would suggest that the imac should last 6 years, you have had it for two years so they should give you 2/3rds (66%) of the original price as a refund (minimum) - don't suggest this to them, let them work it out or offer you a full refund.
2) If you have no luck with the "local store" then you need to write to head office (ask the store for the address) and I would suggest that this will need to be recorded delivery.
3) To help you with what to write etc I suggest you contact Consumer Direct - Practical Consumer Advice http://www.consumerdirect.gov.uk/. There is a phone number on their site and they will help you with the letter contents etc.
Hope this helps, and keep us posted on the results.
J.