Following ZZAJ's comment about extra weight in the back, I have added the steel spare wheel, that I had lying in the garage, together with a quite heavy toolbox and I have to say, the ride is significantly better.
So far I have not noticed any real difference in fuel consumption, maybe 1 or 2 mpg, which I am prepared to accept in the interest of a smoother ride.
Have driven from Solihull to Stratford on Avon, along the A3400, (50mph max limit) this afternoon and recorded 62.4 mpg going out and 58.9 on return, both indicated figures, ( well, it is downhill to the river) which is really no different to running without the extra load, so for the moment the load will stay.