What's the best way of thinning Mr Surfacer for airbrushing?

The best thing to use is Gunze Mr. Leveling thinner. It already has retarder built in. You can also use Lacquer thinner. But use slow or medium thinner. You should be able to get it from most hardware stores. Try not to use automotive grade lacquer thinner, its usally to hot and will melt plastic. This works for the 1000 and 1200 Mr. Surfacer. Make sure you clean the airbrush out with lacquer thinner when your finished spraying.

OK, thanks. Can anyone point in the direction of what Lacquer Thinner is here in the UK

With what you have said I am guessing that Mr. Surfacer is lacquer based? Good thing to know since I would have put it down straight on the model and really mucked things up.

You can use it straight out of the bottle with a brush. The 500 is mostly used for filling small seems or sink marks. It shrinks so you may have to use a couple of coats. The 1000 is used for smaller places to touch up. It can be mixed and sprayed. 1200 is best used for a primer coat. Even thogh its laquer based its very weak. You can use it straight from the bottle. 500 makes a good anti slip coating, weld seams and etc.

I believe it's called cellulose thinner over there.