The build

Big IF, I know, and I don't expect you to know the answer, but, instead of digging down further (if you have too) could you raise the platform up by 75cms?

we did look at that even before this problem came up. As the back of the house is slightly below the natural ground level. Due to slightly sloping ground. Raising the drive to meet it would cost a fortune! We don’t want lots of steps up to the house. At some point we’re going to be unstable on the old legs and not cope well with steps. 
That aging process is being speeded up by incompetent idiots stressing me a little    .