If he is a qualified architect and is using that term architect to describe what he is, he must be registered with RIBA. He must also have professional indemnity insurance and this just doesn't stop when / if he decides not to trade anymore.
Go to RIBA and check him out.
The so-called guarantee you refer to and very likely to be the same as that provided by NHBC or Zurich or the other insurance backed warantees. The 10 year bit will be for the structural components only - in the event that the foundations sink, or the roof structure wasn't designed properly. This particular should have designed by a Structural Engineer and the calculations shown and approved by Buidling Control. Your architects would have overseen this (and probably charged you for the services of the Structural Engineer).