When you say the flowers haven't last long, precisely how long did they last? What part of the country are you in? Is the wisteria in the ground or in a pot, and how long has it been there?

The flowers lasted about 3 weeks and its planted in the ground. We live in yorkshire. The plant is in a really good spot growing against a wall in the sun. We've had the plant for about 5 years. It grows at a tremendous rate and i prune as advised.

3 weeks is about what I'd expect - 4 or 5 weeks is a bonus. Ours came out nearly 3 weeks ago and lots of the blooms have now been battered by the wind, whereas last year, it looked fab for a good 4 weeks - weather conditions were different.
Doesn't sound like you're doing anything wrong to me...

Does your wisteria have a lot of foliage?

It does now, yes - there's a photo taken very recently in my Photos which you can see by going to my profile page and selecting Photos - actually, its probably displayed on the profile page anyway, lower down, as its so recent - taken a couple of weeks ago, the wind's destroyed some of the flowers now.