Weber Q1000 Gas BBQ - Canisters won't screw on

[Shades]

I have a Weber Q1000 BBQ; a few years old, lightly used and well looked after.  I recently had to change the canister and the new one just wouldn't engage with the thread on the regulator.  I had a box of 3 and the other 2 were both exactly the same; the old canister still screwed on fine.  I took the old canister and new ones to a local garden centre and tried them on a new Go-Anywhere BBQ; all of them engaged with the thread.  They couldn't give me much more help but advised that buying another box of canisters wouldn't help and that my box were the right ones.  I assumed that perhaps the thread on the regulator had been damaged (cross- threading) but despite a good look with a magnifying glass I couldn't see a problem.  I decided that the best way would be to change the regulator and hose although, surprisingly, spares didn't seem to be readily available and it did look like some suppliers were over charging for what looked like the correct replacement (794 variety).  There was another garden centre that I thought might be more knowledgable on Webers and may stock spares (ie ensure I got the right one).  I thought the easiest option was to remove the regulator and hose and take it in, however; when I tried to undo any of the fittings none of them would move and I was worried that with more force I'd end up damaging other fittings, or the burner element.  That kind of put the brakes on that repair plan.  Wondered whether that was a safety aspect to stop amateurs playing with gas fittings?  I'm pretty much stuck unless it went to a Weber repair place; not sure they exist?  Any ideas?