iMac repairs

[Dave2002]

A friend has just reported a dead iMac, which he suspects is a failure in the graphics area - not quite sure how he knows that.

Any suggestions for (a) diagnostics, and perhaps more likely to be useful/needed (b) good repairers?

Possibly wouldn't be worth spending more than £300 to get it fixed - but as it's a 27 inch model that could be worthwhile.
The machine dates from 2011.

[Lento]

A friend had a dead iMac recently about the same age: took it to a local Apple shop (this one not called an Apple Store) who tried various things then sent it away. Came back cured, and all for free. Unfortunately my friend has no idea what they did; not the hard drive apparently.

[Nick Armstrong]

A friend has just reported a dead iMac, which he suspects is a failure in the graphics area - not quite sure how he knows that.

Any suggestions for ... (b) good repairers?

Where is he? If in the S.E. these guys have given me great service: http://www.ifixexpress.co.uk

Their guy Ashley came to mine and fixed it at home; ditto, later to replace HD with SSD on the MacBook Pro...

[MrGongGong]

These people are very good and reliable in my experience

http://www.macupgrades.co.uk/store/