Following my threads on pipe sizes and electric water pumps, I have traced the problem to airflow. The ducts on the outlet side were inadequate. Running with the bonnet off has brough the temperature down to a stable less than 90 C so, happy days. I've just got to redesign the ducting to replicate running without the bonnet. I feared the worst but things have turned out okay.

Thank you all who took the time to consider the problem and for offering valuable help

Wow thats the hottest news I've read today!

I was worried how steamed up you were getting about the hole thing, under pressure and venting all the time.

Glad to see you have cooled down now.