On Tue, 18 Nov 2008 15:54:07 +0000, Peter Johnson
wrote:

I'll report back.

Plumber arrived 10 minutes early, looked at the installation,
explained that he would try the tank blow-back method and asked me to
turn off the water preparatory to draining the header tank. Whilst I
was doing that he took the head off the shower, held the hose
downwards and turned the water on - result air lock blown out,
everything now ok. No charge for that so he's coming back to flush the
central heating, which I was recommended to do two years ago.