Greenhouse base installation, battle against the relentless rain

I was kindly given a greenhouse six weeks ago and decided to buy some wood and other stuff to construct a base. Having done so, I decided to wait for a weekend of good weather to put it down. After five weeks, I am still waiting for a weekend of decent weather. Over the last two weekends, I managed to get it constructed, holes dug and the base down, and levelled. I dug 15 inches down which took me into the pure clay subsoil (I should set up a pottery factory and retire), the bottom of these holes soon filled with water. It appears the water table on my plot at the momnent is only about about a foot below the surface.

I tried to pour Postmix around the corner posts but didn't have enough, so ended up with sludge that won't set. I am going to buy another couple of bags and see if I can get the posts set in concrete whilst I still have the base level. Hopefully with some help I will be able to start constructing the greenhouse next weekend.