DAY 3
Well, it being a bank holiday Monday in England... it has been raining on and off all day.  Despite this I have made excellent progress.  First thing, I placed my two 'H' frames in the holes and check how far off level the four holes were.  More digging on all four was required.  This was back breaking work - breaking up the soil with a crowbar, then scooping out the soil by hand.  After about 4 or 5 attempts I managed to get the two frames sitting level.
Then I got the spirit level out and checked all 4 vertical poles were straight.  I used some old bricks and rubble I had to wedge them in position in the holes.  When I was happy all 4 poles were vertical and the two large joists reasonably level, I screwed to cross joists into place to hold the whole frame together.  Luckily a neighbour has lent me his mitre saw - so I was able to get the correct angles on the joist edges.  I screwed them in place with 2 Heco Topix® Countersunk Hardened Steel Woodscrews on each pole (having pre-drilled the the joists to make it easier to screw them in and less likely to split the wood.
Once this was done I had the core structure assembled and level.  Just as another heavy shower started I put the cement in the holes.  I used B&Q Blue Circle Postcrete.  I half filled the holes with water, then emptied the bag on top (one bag each hole).  I put this on top of the bricks and rubble I had placed in the holes.
Within 20 minutes the structure was solid.  Once again, I decided to quit whilst I was ahead.  Next weekend I'll start cutting and screwing remaining joists and the decking in place.  Really happy with the weekend's work - I'm pretty sure the most difficult part of the construction is done.  Amazingly the hostas, fern, soloman's seal and iris under the platform survived!!!