Fuse/Cable Relationship

I have used a redundant 6mm shower cable to supply power to a new immersion heater. The cable was protected in the CU with a 30A breaker. Should I reduce this and if so to what?. Also would it be advisable to install a switched fused spur to isolate power to the digital immersion timer. Any help appreciated.

So long as the redundant cable is sound and has the required insulation resistance and cpc ('earth') continuity it should be usable. (are you able to check these?) Your circuit protective device should be rated larger than the design current (the current drawn by your immersion heater) and smaller than the current-carrying capacity of the cable. A 30 A breaker suggests it's either a BS3871 or a plug-in fuse replacement. If it's the former you may find it impossible to replace, but a 6mm cable should carry more than 30A in most domestic situations so you can leave that as is. (Although, ideally, you'd replace it with a breaker closer to your design current.)

You should have local isolation at the business end. A fused connection unit not only gives you local isolation and fusing, it also allows you to de-rate and connect the heat-resistant flex you'll no doubt be using between the fcu and the immersion.
(If in doubt, get somebody to check it all out for you professionally)

Thanks dingbat for the quick reply. As you suggested the breaker is a plug-in fuse replacement. Will follow your advice and thanks again for your help.