Bongo Fury

We had the bleeder hose connected to funnel as if in bleeding and the water was hot.

Will do test again when running correctly.

Should have added that you really need to have driven the motor with rad cap on for few miles to force any gasses into the coolant and allow pressure to drop first before removing cap, obviously.

Thanks Bongolia

Working with a mechanic took out the water pump we both thought the pump looked good. Put the new one in then everything started to work correctly. Had bleeder to funnel for a very short time had bubbles come up in water tank not in funnel.
Thought had head problems.
Took for long drive no more bubbles left overnight checked water tank topped up. Took for another drive no bubbles and very little drop.
Took for longer drives perhaps a few hours with breaks in-between.
No drop in water tank.
Bongo running wonderful since ........ it's purring along.
Very strange about that water pump somehow was intermittent over several months until stopped working just can't figure it out. Blades looked good! Belts were tight so that it turned.
Robert.

Weird.
The impeller may be an interference fit and could loosen when hot which could explain why the problem was intermittent.
It would be interesting, if you are able, to clamp the pulley side in a vice and using a screwdriver in the impeller trying to see if it would rotate.Just a thought.
I was rather expecting you to say that the impeller blades where corroded away!
Anyway it is sorted and the heads good.

Interesting one. Always useful to have the resolution posted for future browsers.