I'm in a similar situation, but worse. On a Claud Butler Urban 200, the weld at the joint of the seat tube and BB split totally just before Christmas. Bike was bought six years ago in a different part of the country, cannot find receipt - not with manual where I'd left it. Falcon (who own the Claud Butler brand now) are looking into it.... They are quite within their rights/obligations to tell me to go away, so anything beyond that is a bonus. Currently, they are investigating by looking at photos and are civil on the 'phone.
Really galling- everything else bar the cranks and front chainrings are in perfect working order. If I end up having to replace the frame myself, where's a good place to buy one from and what vital statistics do I need? The wheels are 700mm and the forks have 28*250 stamped on the tube inside the headset.