Update
Well I switched back to the 17.5inch wheels and sold the 19.5 ones for a criminally low figure. I put a set of Kumho KDR03s on the back and stuck with Michelins on the front. The handling was improved a good bit and it did track straight on poorer surfaces.
Here it is with the original wheels. I also stuck an awning on it for our Summer holidays in Spain and France.
However it was still "lively" on bumps and tended to over-steer when initially turning into a bend. I noticed that the front axle was very close to the bump stops and that there was room for it to move further without hitting anything serious. So I decided to do a little surgery on the bump stops. I pulled them out and cut them in half and I also stiffened the adjustable front shocks by a 1/2 turn (now at 2 out of 1.5 turns available). This made a significant difference to the liveliness at the front and it also all but eliminated the over-steer, I'm guessing it was bottoming out on the front stops when cornering and causing the over-steer.
Having (temporarily) addressed some of the front problems I was better able to diagnose that the rear springs are too stiff for the load - That's another days work and taking leaf springs out in the middle of a French campsite would probably have been frowned upon.
So I think where I'm going (Yes, yes - I know - in circles) is to put a pair of air bag front and back and remove one leaf from the rear. The challenge is getting bags which are neat enough to fit but also have the amount of articulation necessary (160-180mm).
The only other small issue on this shakedown holiday was the fuel consumption. About 14-15 mpg (imperial) or 19L/100km Ouch!!!
On the plus every thing else work as planned. The 3 week holiday was great.
Here's the beast about to disembark in Gijón, Spain.
and a couple of idyllic shots in Spain