Was shopping aroung for a greenhouse and came across this. It is cheaper than any other site i came across. Even cheaper (including the shipping) than any of the garden centres.
The reviews seem pretty good too. There is some negative reviews around the cover, that the plastic can come away from the zip so you maybe need to handle it with care but that seems to be quite common with plastic covers.
Amazon sell them too for £16.99+£4.95 S&H if you have any vouchers that you prefer to use.