Hi I have had a better look now I am home (i.e. taken it apart and red the number on the side of the gear rather than a brief look this morning) and it is a standard 81t/48dp spur not 100.
This makes it all a bit simpler. I found this post
http://www.rctech.net/forum/electric...4-gearing.html talking about the same set up that I have.
I think that a 23 or 24 pinion is the safe way to go to start with as the tack does not have many straights so top speed is not so important. People on the other forum have talked about going as down to 21 or up to 26.
Thanks and if anyone has any comments on this it would be good to here them.