Since you guys are in the UK (less expensive shipping) you should try to get the Extreme Duty diff and layshaft gears from HobbyThek. I bought them for my Firehammer, got a small alloy engine mount and have not had any problems since. The gears are much tougher than stock FG and Smartech gears. The teeth are "regraded" to allow more slop in the mesh without causing problems. Check them out here (you will need to scroll around on this page):
http://www.rc-car-online.de/neue_produkte_eng.htm