Thanks very much for your replies. I have since looked at Tek Tanks catalogue and they make what they call a GWS (grey water system) tank which contains a pump control system, sensor and relay to operate a discharge pump situated outside the tank. They recommend a "diapgragm" type pump with a large bore 25mm - 38mm to go with this tank. But I have just spoken to them and they also supply a tank similar to the Whale tank mentioned, made in USA by Attwood (with float switch and pump inside). HOWEVER, the man from Tek Tanks said that grey water must not be put into a holding tank used for the heads because the soaps upset the bacterial action and cause blockages. I must therefore rethink. I don't really want a separate grey water holding tank. I think that maybe the Attwood tank and pump (about Ł80 plus vat) will be ok for the shower and heads basin - with a loop back to seacock, and galley waste can go the same way unless it proves too much for it and blocks it, in which case it can go directly overboard below waterline.