Yep, It sounds like the circuit breaker on the consumer unit is either wrong spec, ie youve uprated your shower and not the breaker or its failing and needs to be replaced. PS if youve up'ed the shower, say from 7.5Kw to 9.5Kw you are also likely to need a heavier supply cable too! A Fire is a dreadful thought. Keep safe, Call a Pro.
Uncle Panda, June 2009