thanks for the replies folks there is a bit in the garden that is sheltered from the prevailing wind by shrubbery in next door's garden so that might help
McGregor polytunnels very kindly did me a drawing and quote for a mini version of one of their agricultural polytunnels - a variation without the mesh sides as they think open ends should be enough ventilation for such a small one (approx. 4.5m x 6 m) Marquees of a similar size work out much cheaper, however the McGregor polytunnel would be on a strong frame. They did say that there should be little concrete pads that it will be bolted on to and that it could be taken down and put up again. Hmmm - will have a think about it.
would be good to hear from anyone who's used a McGregor polytunnel or similar
have also had a quick look at 'dancover' who make similar shelters
thanks again!