27" iMac 12,2 (2011) Price Advice

Hi All and thanks for your comments in advance,

I have been offered the above iMac with the following specs. This will be my first iMac, so I am a bit unsure on second hand pricing.

Details are as follows:

27" Mid-2011 iMac
Intel Quad Core i5 processor 2.7
12GB RAM (upgraded from 4GB)
1TB 7200rpm Hard Drive
AMD Radeon HD 6770M with 512MB

All in good condition.

Can anyone please give me an approximate value of what this is worth (preferably in £) before I make an offer.

Thanks once again in advance

Thanks for your input.

Managed to get the machine with 2 years Applecare for £800 and extremely happy with it!