Alan, have you tried bleeding the pump by unscrewing the large silver screw on the pump itself while its running? Don't unscrew it all the way as it'll fall out and you'll have water everywhere. Also have a look to see if there is a single pipe in your immersion tank cupboard which appears to be going nowhere .... is there a cerrated screw on the end of it? undo it and you might get some air out of that. Keep a towel handy.
Chris
October 2007