Hello Geoff & John. My dad has now got a bit more detail: (1) The
boiler is a Potterton Profile (no model no. as yet) and is situated in
the attic/loft. (2) We have a sealed system - but it does have vents at
the top of the boiler (3) no visible leaks. (4) the pressure is not
dropping - it is currently set at minimum. (5) we do not have a header
tank. (6) this whooshing sound is ONLY present when the hot water is
ON. There is NO problem if the central heating is switched on (7) these
sounds come from the hot water tank.

I hope this helps you to offer a bit of advice.

Warm regards
denise )