Like all the other guys, congrat's dude!
The Prometeia holds temperatures slightly better than the Vapochill LS and it's less messy to set up too. If you're into modding, you can swap the coolant from R404A to R402A, that shoudl give you about a load temperature about 5 degree Celsius lower.
As for CPU kits, sorry I don't have any spares... I bundled it off with my second Mach II GT when I sold it earlier. As for stores that might have them, I would try Extreme Prometeia
or MIPS Computer
. There are stores in North America with stock but shipping to the UK might be very expensive.
Good luck, a couple of my friends in Europe say Extreme Prometeia is an awesome store to deal with so I'd try them first.
PetrolHead no need to recharge witih R404A, it comes standard with that refrigerant.
R507 won't give you any performance increase, R402A will slightly.
I hate to be the bearer of bad news but your Winny might not like the cold.
Some Winchester chips have problems running sub zero. That gives you a reason to upgrade though.