You can use the T7200, T7400 and T7600 - all socket M.

Socket P CPUs (T7x00 with x = odd or above 6, plus others) won't work.

thanks any idea what stores in the uk online or not sell these??? or do Dell sell it direct???

None of the socket M CPUs have been produced in a couple of years  -- you may have to look for a used one or to a shop that has some old ones still in stock.

I doubt any of the major vendors - Dell, newegg, tigerdirect, PCConnection, etc. will have them.

The T7200, T7400 and T7600 are the fastest ones you can use -- they haven't been produced in some time, so look for a vendor of used CPUs.