Thanks very much.

Someone told me that the foam is largely from laundry detergent that comes out of people's swimming costumes so I wasn't too worried about it at this stage. My brother in law adds a liquid that just eliminates the foam but as you say it obviously can't remove the substance.

Water in the South of England is generally hard and scales up kettles, washing machines etc so I can see that it would affect the heating element of the hot tub.

I would expect the sodium bicarbonate to solve that though.

Thanks for your help.