Hi Stuart
My knowledge of microchipping tracing (and I might be wrong !!) is that there is a company in the Uk that has all the details on their system for each particular dog. These details would have been entered at the time of microchipping by the then, original owners. If you loose you dog, they can be scanned, which will provide all the details to return it to its home. Thus giving you the address of the person who originally had your dog, to contact for breed lines etc. But I think with the current Data Protection Act, they would not give you that informtation. If you know who bred your dog, and what its registered name is, then you could try calling the kennel club, and they might be able to provide you with a 5 generation pedigree certificate.
Sorry just re read your post and saw that he was bought from a pet store., so not sure if any of the advice would be any use !!

:D
I'm sure someone would be able to give you a starting point...
Natasha