Firstly, thanks for all the replies.

It should be noted that my motor/cooling system does not suffer from any overheating problems.

I guess I just wanted to know if I could close the heater valve off without suffering any longterm consequences whilst trying to lower in car cabin temps when I'm in stage. I will be going back to my state scrutineer for clarification on the heater box/demister issues because if I can get rid of the heater box I will do so just for the weight saving alone. I asked the scrunineers about bypassing the heater core but they were not that keen on the idea but like most things to do with rallying I guess its just one mans interpretation of the CAMS manual compared to anothers.

Feral - my motor has been through the building prep steps you mentioned above, unfortunately I am a perfectionist with access to a large machineshop and I can be my own worse enemy so I was maticulous when doing all the work to my A series. Reg Cook was the one who informed me about the simple water pump tests you have mentioned. I have performed similar tests some time ago after I modified the water pump vanes to prevent cavitation problems.

It looks like it's another visit to the scrutineers to find out more.

Thanks again to everyone who has helped,

Paul.