Well I got answers via email, the phone line I was given to ring in Le Havre seemed to be on answerphone permananetly. I think the "restrictions" are only on full sailings, so as long as we book before the boat is full we can use them on peak dates. So for us definitely using 6-8 tickets at peak times this year, it more than pays for itself, but the extras for extra passengers do stack up. Living in the west country, hauling over to Dover and back down to Charente Maritime is a pain, it seems relatively easy via Portsmouth with a good karrimat and sleeping bag...... and the diesal saved pays the increased cost over the tunnel/Sea France. The great thing is that spare tickets can be passed onto friends and family, when I looked at the tunnel scheme, whoever buys the tickets has to travel, not good for us as there may be times I go without DH!!
Unfortunately, it looks like another trip this year will have to be squeezed in