Do I need a stronger wheel?

Thanks, mate. Suggestions on stiffer rims that aren't exorbitantly expensive?

The guys at the shop checked the wheels before I bought them and they were pretty much spot on so I don't think it was bodgy to start with.

No doubt it was fine. But a wheel built fresh is like anything else on a bike and will take some bedding in. Most wheels benefit from a service after about 500 miles.