Many thanks Jonathan. Fairly certain the water is caused by over pressuring of the tank, in turn causing leakage from the cap on the tank – the pressure was so great at one point that water was also leaking from the filler cap fitting on the transom.  What I don’t understand is how the tank is becoming pressurised.  If the breather (if that is what it is) was blocked, that would cause a vacuum as water was drawn from the tank and I’d expect that to result in taps/shower not working …….. unless it is a duff sensor and that is causing the pump to pressurise (every 2-3 mins) when there is no demand.  I will start with an air leak search and then move on to a strip/clean/repair of the pump/gasket/strainer.

    As an aside, we are finding the transom shower a godsend this summer, as we don’t have an internal shower (previous owners removed) – exploring the S Coast / Scillies / Channel Islands and it’s definitely warm enough for a swim! Thanks again. Chris