For gaming stick with the GTX560ti. It is the prime contributor to performance.
A GTX560Ti needs 500w with 30a on the 12v rails plus two 6-pin PCI-E power leads.
If you can find a quality 500w psu that is significantly less expensive, then you might supstitute. A 600w psu is strong enough to run a GTX580 or even a 7970.
How many motherboard expansion slots will you possibly use? For most of us, it is just one, the X16 pci-e graphics slot.
A full atx motherboard, like you indicated has 7 expansion slots. A m-ATX motherboard will be smaller, and cheaper, with only 4 expansion slots. That should be sufficient. It looks like you will be connected via wireless N. If so, you will need one slot for that. If you do not need wireless N, then the motherboard lan will suffice. If you will not be using a "K" suffix cpu, you do not need a Z68 based motherboard. A H61/H67/P67 motherboard will usually be cheaper.
A case is a good place to econimize. Any case with a 120mm intake fan will privide adequate cooling for your cpu and graphics card.
Look at the Antec 100-200-300 cases. They are well built, with good ventilation, and are inexpensive. On a real short budget, I would even look for a cheap noname case, or perhaps a used one.
The stock Intel cooler will be fine, even for a modest overclock.
DDR3 ram is fine, no real benefit from faster. But, since ram is so cheap, I suggest a 8gb kit(2 x 4gb) of the cheapest you can find. No need for fancy heat spreaders.
Hard drives prices are crazy today. Consider buying a ssd for the os and a few apps for now, and getting a hard drive for expansion later when prices are more reasonable. A ssd will make a major performance difference in everyday work. A 60gb unit can hold the os and a few games. 80gb might be better if you can handle the budget.
Look first to Intel or samsung for reliability.
Thank's for the reply's, I have a budget of £800-£900. The total cost of this build comes to around £850. Would prefer to get it under £800, in terms of usage; the two main uses will be gaming such as battlefield 3, and the other use will be for a game developer course i am starting.