should i get hold of a cheaper fibre glass or a very expensive carbon fibre??

That depends on how much your cello is worth, how much you'll be travelling with it, and so on.
More details on either of the above?

my cello is around 400 pounds and i travel by buses and trains a lot.

It also depends on your physical build. Glassfibre cases can be quite heavy in themselves, while a carbon fibre case is so light it almost feels as though you are only carrying the cello and not the case.

A friend of mine (professional cellist) says many of his colleagues have opted for thickly padded soft cases. It keeps costs down while still offering good protection.

If you do go for a carbon fibre case, shop around, as there are lots of different makes out there and the prices vary a lot. Headcase (based in Wales) make a carbon fibre version of their glass fibre case for about £700. Gewa produce one for about £900, but if you go for one of the top of the range brands, such as Accord, then you'll be looking at £1200 upwards. By comparison, glassfibre cases produced in far eastern parts of the world start at around £160.

As cellocase has said, a lot depends on how much travelling you will do and also the value of your cello.