Rough price for concrete base?

[pattieG]

Planning to build a shed to put my home gym in soon. The base will be 3m x 3.75m and consist of 100mm MOTY type 1, DPM and then 100mm of C25 concrete. I was thinking of doing it myself and have priced it up at around £350. I was thinking of getting someone in to do it for me though if it isn't going to be too rediculous so any ideas on how much a groundworks company would charge to do this?