What tyre pressuse should i be running in my buggy i ride it on longish grass some times sand there standerd size tyres and i weight about 9.5 stone???

chears

Tire pressure is always a compromise thing, rolling resistance vs. traction. i.e. Higher pressure = less rolling resistance but less traction. Lower tire pressure = More rolling resistance, but better traction. So ideal tire pressure is constantly changing with the strength of the wind - the size and type of kite you're using - the type of terrain you're on - and even the style of riding you do. But here are some suggestions:

Smaller kite + higher wind allows for higher tire pressure to take advantage of the higher kite speed.

Lower wind + bigger kite requires lower tire pressure because of the increase in side pull.

Grass, dirt, and hardpack, and softpack sand require very different pressure requirements, and very different considerations (especially sand)

Sand acts much different than any other riding surface because of its tendency to deform under weight. Because of this, rolling resistance will begin to increase as tire pressure gets past a certain point. (it takes much less energy to deform the tire than it does to deform the sand) On hardpack sand a good method for achieving ideal rolling resistance is: Coast in a straight line while not under power. Now look back at your tracks. if your tires have made an indentation in the sand, your tire pressure is too high. Your tires should float over the sand when not under a side load.

Softpack sand requires ridiculously low tire pressure and/or big fat wide tires (I use 24" tall - 12" wide tires running at about 5 psi when riding dune sands)

Running at too high of a tire pressure is hard on your equipment (and you) because it reduces the natural vibration dampening that the tires give

And finally (in case you haven't figured it out yet)...Carry a pump and tire gauge with you.