At last I can raise my head from the distractions of earning a crust.
Sorry for the necessary radio silence, and thank you for all the comments and advice.
mixture - The GeForce 7600GT graphics card is basic but does everything I need as a non-gamer and the image processing software does not support offloading to the GPU anyway - though I like that you thought of this, thank you.
My W7 x64 Pro licence is not OEM - it is a System Builder licence that I purchased for a hundred quid via Amazon. I will try to ask nicely that it be allowed on the new mobo, but if not will of course have to accept paying for a replacement.
Interestingly given Spitoon's comment, I've since had several personal recommendations to go with a Gigabyte motherboard. One of these recommendations was specifically for their Z77-D3H on the basis of its bang for the buck. I then followed up Milo Minderbinder's suggestion to contact DVC - and blow me if they didn't recommend exactly the same motherboard. They also recommended the i5-3550 3.3Ghz quad core CPU, but I've had another system builder and a digital photographer instead recommend the i5-3570K 3.4Ghz CPU, the K indicating "designed for overclocking" with up to 4.4Ghz being commonplace. The software I use supports multithreading and the indications are that I will get the massive performance increase for the task in hand that I seek if I go down this route.
Armed with this information I did some browsing and found
bundle that appears to be competitively priced especially as it includes 8GB DDR3 Corsair 1600Mhz RAM and a decent cooler.
The price is considerably less that DVC's quote for the lower-spec i5, only 4GB RAM, and without CPU cooler. Comments welcome before I see what room exists for negotiation!
Cheers,
TLXV