I had a similar problem with a mytyres fitting centre in Chesham...in my case they could not fit the tyres as their machine had broken and it was still broken a week later...to be fair, this was during the snow earlier in the year so that may have delayed getting the machine fixed....eventually went and collected the tyre, got a refund for fitting from mytyres and got WIM in Chesham to fit tyres for me.....the original fitting centre was a bit like the one you describe and I am not sure if I would have trusted them with my AMG alloys!!
how much did WIM charge for fitting? I bought tyres for my Boxster from them, and they were excellent.
For the Falkens, they were about £40 more each.