Wisteria sinensis can take up to 7 years to flower, but its best to train and prune correctly as it grows as well - this helps with forcing flowers. They also prefer a fair bit of sun, so it strikes me it's possible it's not getting much sun against a north facing fence, is that the case?

Thanks for that and, yes, I'm afraid it doesn't get all that much sun. Unfortunately it's the only position I can put it in. I'll try and find out how to prune it - it's certainly got plenty of support and is growing well.

Here's a rough guide - pruning should be done twice a year, once in August and again by mid February - training usually consists of deciding whether you want one, two or three main stems going vertically, then taking out the top once its reached the right height, and then pruning is carried out on the lateral (side) shoots which grow horizontally from the mainstem. Removal of other shoots which arise from the base or half way up the mainstem/s should be done regularly too. If you have a very large area in which it can ramble away, it will flower all by itself (see my Photos, there's a picture of an old one which never gets any treatment at all, but you can see how much room they need if left unpruned).