Brake Light pressure Switch

Their are some on ebay but which one do I need 3/8 UNF or M10 x 1
or are their better ones ?

Unlikely to be metric on a Series II

You talking about a brake light switch, or the differential pressure failiure warning light?.

Its the in line brake pressure switch, which activates the brake lights. I stamped on the brakes hard a couple of times today and they eventually came on. I have read on a thread that these items dont last very long ! The ones on e bay are about £5. Are there any better ones available.

Vauxhall's and some honda's use the same idea, I doubt the threads will be the same though, but they are probably better being oem.
Have you considered fitting a pedal operated switch, its only a couple if wires and the pedal box may allready have the cutout for the switch.