I can get pro seal RTV for £5, I think that's my best bet.

Permatex is going to cost me £13 with postage.

It looks like the red hi-temp RTV is the best to go for, it withstands 370c compared to 315c of the normal stuff, and is approved for repeated hot and cold cycling.

http://proseal.supergluecorp.com/80724.html

I just bought it, so I hope it's ok