Once again thanks for all the ideas. I did a bit of internet research and came up with the idea of making a T shaped one. It's made from 2 pieces of wood, which I slotted together glued and screwed into place. I then partially hollowed out 6 areas on the wood for the tripod feet to sit in and added 3 sets of wheels (off an office chair). It weighs about 3-5kgs (6.6lbs-11lbs), don't know how this compares to dollys that TV companies use.

There is a video on youtube abouying making your own dolly, I can't remeber where but a search there would bring it up. It looked a bit ropey, but functional and you'd need a very smooth floor!