Stu,

Concrete is approx £100 m3.

So if you did you base 100mm thick it would be 3m3 - £300 ish quid. However as it will be a part load (6m3 to a lorry) you may get charged more.

I would make it 6" deep and do it your self. It really is easy. Just don't forget to put a bit of a fall across it to stop standing water if it is outside.