Chris
If it is the same heater as we use it will be leaking from where the probe sits in the centre of the heater. We have found a plumber who will take it apart (it is very tight put some ptfe tape on it afterwards) and silver solder the join and then its fine for about 12 months before it starts to leak again. The design is poor all that they have done is crimped the end so with expansion of hot and cold it eventualy starts to leak again.
Hope that helps
Ray