We got a builder in to do the conservatory but I did do quite a bit of the work myself. The concrete pour wasnt really difficult. Started at the middle and worked out from there. The hard part was walking about in the concrete while pouring it and trying not to fall in it!
How close are you building to your boundary? Can you build the retaining wall at the boundary and leave say enough space to lay a row of 450mm slabs around the workshop for maintenance access?