I have an old water heater (providing hot water on demand, not filling
a tank). This water heater is old and inefficient and needs replacing
as it has started leaking and finding the replacement part is
difficult.

Anyway I reckon it is time to get a new one, can anyone recommend a
good water heater, wall mounted, providing hot water on demand for 1
bath, 3 sinks and a washing machine.

Thanks,

Joesph