So, my ideas are coming along nicely but i need a source of pneumatic tyres. Buggy shops sell them for 15 pounds, but i want to spend less.

They need to have an integrated bearing, will be carrying 13 stone + have sideways / forward forces. Size wise i'm thinking about .... 4.80/4.00-8 (which is copied from a web site - i'm guessing inches, no idea what it means!)

Standard kite buggy tyres are just ordinary wheel barrow tyres, these come under the classification of small implement tyres which you should be able to buy much cheaper from your from your local tyre centre, failing that try a builders merchant, garden centre or an agricultural supplies dealer.
The size 4.00/8 is a 400mm total diameter that fits on an 8 inch rim, the larger sizes read like this, 16-650/8 - which is 16 inch total diameter, 6.5 inches wide to fit an 8 inch rim. Wheel barrow wheels are ok as they are designed for bearing heavy loads, so 13 stone distributed between 3 barrow wheels will be fine, if they don't come with the bearing size you require they can easily be changed for the size you need.
I hope this info helps, good luck with your build.

Try a couple of these . They look sturdy enough to carry 13 stone.

great minds and all that.

Already started looking up wheel barrow tyres. Not many places seem to sell them.

That one looks interesting.

Try these links; http://www.rosshandling.co.uk/wheels.asp
http://www.wheelbarrows.com.au/cattleprod/products/W2200SPA
http://www.haemmerlin.ltd.uk/spares.htm
http://www.servicecaster.com/index.shtml
http://www.northerntooluk.com/product/default.asp?
http://www.northerntooluk.com/product/default.asp?id=174&catId=40&subCatId=302

I hope this brings you a bit more joy.

two buggies together is one of the motivations for this. Just call me mad! (plus i've got complete trust in the psychopath Chryzi who introduced me to flying on the grounds she hasn't killed me yet).