If the cable was fine with a 20A MCB before it will still be fine. A 20A 2.5mm radial is pretty standard. No need to waste your pennies
The 3A fuse (or whatever size it is) in the switched fused spur unit will protect the central heating wiring.
cheers Spark123. but i may not of explained it very well!! my fault!!
Basically the immersion heater was fed by the night rate meter, through a timer and 20 amp breaker. i am having this meter removed and need to still use the supply to theold immersion heater to feed the boiler, via a FCU.
i need to wire the feed back into the standard CU and need to fit the 10A MCB into the CU to supply the boiler. Does that still sound ok??
cheers
george