200£ For a 7950 ? that's too expensive even for it as brand new, I can get a R9 280 at 200€s heck even R9 280X for 230€sh with an Aftermarket coolers example :
http://www.rueducommerce.fr/Composants/showdetl.cfm?product_id=4911025%23xtor=SEC-802-GOO-[Composants]-[RSYS:31:4911025]-49598307014#!moid:MO-F76E2M27248721
I found some second hand 7950 at 150-170€ the only problem with them is they are pretty rare
I'm in the same situation as you and I'm considering all my option, I am investing little by little but more and more on mining.
You best deal for now would be a second hand 5850 @ 50€ it should get you around 350Kh/s
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Once you have enough money jump the both to something more effecient such as the 79xx or R9 280 or R9 290 (for now the X version is not really worth it, maybe with custom cooled version the difference might be noticeable).
One last as you have one slot and your don't want to invest in new hardware but only if you have beffy PSU try checking for HD 5970 I found one at around 150€ (second hand of course)