I suspect what's happened is the grafted upper part which was the cultivar 'prolific' died, and what grew next was off the rootstock, which is likely to have been the basic Wisteria sinensis, so those are the flowers you're seeing, and that's why they're not as impressive as you were expecting. Have a look at the link below, see if you think the flowers in the image look like yours

https://www.rhs.org.uk/plants/19126/Wisteria-sinensis/Details

This happened to one I was bought as a birthday present. it was stunning then 2 yrs later it was a pale mauve. When I looked closer the flowering shoot was below soil level so below the graft. My other one rarely flowers so it is coming out this year.

Bamboo, thanks for your answer, makes sense but wondered whether I would be able to see this as the main trunk is just one branch no other growth from the base of the plant, would it appear in the form of a sucker like in roses ?

Sometimes just one dormant bud will take over and the rest remain dormant.

If at some point in the dying back and moving around it was reduced to a short stump, the 'Prolific' part of it may have died off or been pruned out. If it was purchased as a Wisteria "tree", the graft point may have been as tall as the "trunk".

It wouldn't look like a sucker now - a shoot coming off the rootstock a while back could easily have been mistaken for the variety you bought, the leaves are more or less the same - you'd only have noticed it wasn't if you inspected at the time and noticed it was coming from below ground level.