I now accept that my holiday needs to be longer than originally planned. However I am not planning on going peak season. I paid 70 EUR a night for a pitch back in 2009, in the Loire valley. I have done 4 different crossings to France and the tunnel is by far the quickest. I would however book the cheapest and plan around that. I know some caravanners are happy to overnight on the motorway aires, personally I wouldn't.
I am now looking at taking the trip over 21 days, and as my good lady is starting a new job, that has to be taken in to consideration too.
Thank you for your input