So I've been looking around for gearboxes that could take a larger motor
Found this thread on the AU forum ... c&start=90

This is a set of gearboxes on the Vex robots website. ... etary.html

So I put together an order but found that they are only available on the US site and not the UK [page not found if you try shipping from UK]

I contacted the US company who did say that their US shipping to UK is pricey but have suggested that the UK distributor might be able to have them shipped as part of their stock order
Contacted the UK distributor who is busy for the next couple of weeks but will be looking into it
Will keep you posted