Many thanks to everyone for your helpful comments...
This is the first two-stroke I have bought from an importer - I've had all my others pretty much from new, so these problems are a first for me.
I actually bought the bike with a one month warranty but I have my doubts regarding their ability to sort this one out, as I don't even think they have looked at the carbs before selling it.
It looks like I'll be sorting it myself, so thanks again for your input. I'll let you know how I get on with it.