Thanks all, great help! Chrisgixer, if you think I'm going to bet even a pound on the competance of my local VX dealer then you are mistaken

- the majority of the time they seem adequate at best - yesterday they had trouble understanding the difference between a heated rear window and electric rear windows....... and the fact that there might be a software upgrade for the car available seemed like an alien concept to them......
I think I'll get the wheels properly balanced as a start, try again and then investigate alignment after that. Afterall, even wear on tyres can send them slightly out of balance, perhaps they are just more sensitive with load. If it still happens I will indeed weigh up the time and money spent investigating further against how often the car is full.
For the record the tyres are the standard 235/45/R17s and I put them to near the fully laden specification for the trip (the kids are quite small!). I'll reset them this weekend when I get a chance. Definately a great car for a long continental trip, especially on those empty French motorways.....
All comments much appreciated, I don't get much time to do anything to my cars these days (hence the use of the VX dealer) but I do like to have other opinions than my own before talking to them as, as was mentioned in one of the posts, they can follow a procedure but not diagnose an unknown issue from the symptoms.
Cheers all.