I have a 20 amp breaker in the main fuse wired through an rcd the 20amp mcb feeds 3 radial sockets in the next room and a pump out in the pump house all done with 2.5mm twin and earth but I need to put a socket and a light out in the pump house. Would it be OK to use the cable that's supplying the pump and use it to supply a small sub board with 20amp breaker for the pump and the socket and 10 amp breaker for light

Hi,

Forgive my ignorance, but I cannot understand what you are talking about.

"I have a 20 amp breaker in the main fuse". You have fuses and breakers installed in the same panel?

"rcd"what does that stand for?
20 amp "mcb" - - does that mean "Main Circuit Breaker"?

What does "3 radial sockets" mean?

with 2.5mm twin and earth What are you trying to describe? Are you talking about the cable being 2.5 mm or the individual conductors?

Are you here in the U.S.A. I ask because you refer to cable by milimeters instead of AWG sizes.