I've just fitted a new 10.8 kw shower and and upgraded the mcb to the required 45 amps. The installation instructions recommend fitting an RCB to the circuit, but because the CU is quite old, this would have to be external. Is this simply a case of cutting into the circuit adjacent to the CU and fitting it there? (This may be a daft question, but I've no experience of RCBs)
sorry to be the bringer of bad news but you should have not carried this work out.
Because it's in a special Location and it's a new circuit it becomes notifiable work. (both apply)
So you should have either notified Building Controls prior to you doing work or employed a qualified electrician to do it for you.
This circuit needs to both comply to Part P of building Regs (See project pages on Part P: http://www.diydoctor.org.uk/projects/partp.htm) and BS7671:2008.
This may well make your housing insurance invalid.
That being said, now it depends on the type of CU you have, you may be able to replace MCB with RCBO which will serve both as MCB and RCD.
But get a Sparky in to check it because the circuit will need inspection and Testing Certs.
Sorry, I wasn't clear enough in the original message, but I only replaced the shower with a more up to date, and powerful, one, the circuit is the original one.
I don't think you can get RCBOs for plug in type CUs, but I'd be quite happy to be corrected.
had you made sure your cable was the correct size for amps for shower you can't just replace a shower without calculating your cable size. You can replace equipement like for like but this is a special location and the shower is more powerful so not like for like anyway.
I recommend for your own/family and property's safety you get an electrician involved.
Also the fact that you have changed the fuse/breaker rating in theory it then becomes a new circuit. If the breaker is not rated correctly to the cable this then becomes a fire hazard!
Please take notice of the dangers!