Many thanks - I went for it and swapped the heater tank this morning. The heating elements on the new heater were both about 12 ohms, and I was able to confirm that one of original one's elements had indeed blown. I reckon when it blew the element must have briefly contacted the heater tube and tripped the RCD.
The swap can be done with the shower in situ (with power off!) - the only fiddly bit is a clip on the tank that locks underneath a corresponding clip on the rest of the mechanism but it can be done with some careful 'easing'.
The new heater tank was a slightly different design to the original but all instructions were included.
All working now for about £65 and about 20 minutes effort.
Dave
April 2007