I'm building an MK GTR with an R1 engine. Does anyone know whether the standard water pump on this engine will cope with the extra water volume (e.g engine in back but water rad in the front..) or whether it would be better to fit either another, or an extra water pump?

Ive never heard of anyone having problems with R1s in cars so unless you're planning some humungous twin rad setup pumping it all over the car, I cant imagine it will be an issue. If it was you could always run one of the Davies Craig pumps to supplement or replace the existing pump if necessary.