In article om,
says...
On Sat, 05 Nov 2005 15:46:53 GMT, mrcheerful
. wrote:

If I push my engine hard for any length of time, as on the upper
reaches of the Llangollen in the tunnels, it develops an air lock
in the top of the skin tank and overheats.

Might this be a sign of water pump trouble, or is it summat else?

Think logically, how is the air getting into the system? Head
gasket is the most likely, get it tested.

I suspect this is boat in which case the cooling water is taken and
returned to the body of water in which the boat is sitting.

I don't see how it could be anything other than boat. Lots of boats use
a sealed circuit with a heat exchanger for cooling.