Hi.....looking for a little advice on a over heating issue on ford ka

Its a 1.3L, 58 plate, no air con.

Car starts, and runs OK, internal heater starts to warm car, radiator and expansion bottle both fill with hot water, electric fan on external radiator works.

Not using any water.......after 20 mins round town driving dash warning light comes on, put internal heating on full (open a window) and after 20 secs or so warning light goes off and stays off as long as you keep heater on.

Warning light comes on if driving motorway speeds, just takes a little longer then goes off again when you put heater on full.

Any ideas please?