I also had recently to replace the immersion heater in my 2001 331. Thanks to Richard for recording his experience. I obtained my replacement element from JPC Direct. Brundall, Norfolk. The part was supplied within 2 days of placing the order and I saved carriage charges by also ordering a Sidepower Bowthruster Anode. A very useful and recommended supplier!
I separately ordered a 41mm box spanner – but this proved unnecessary since I managed to unscrew the element with a sturdy adjustable spanner applied “end-on”.
However, having replaced the element, I am still unable to achieve hot water in the heads/shower compartment; it always only tepid and needs to be supplemented from a hot kettle. There is a temperature regulation valve at the outlet of the calorifier that mixes cold water with the hot output. No matter how it is adjusted, the water at my taps is always much cooler than at the tank. Presumable the pipes were not lagged but other owners seem to have no problems. Any ideas?