Ring Blackcircles and kick off. Blackcircles are the customer as far as the garage is concerned, but the garages often overlook the fact that you chose to go there and that how they treat you depends on you reusing them or not...
Never use that garage again and leave a shitty review when the opportunity presents itself.
Blackcircles do say to arrive 15mins before slot time. Never had an issue with them but I already had a good relationship with one garage on they're list, so stuck with it... they do all my MoTs.
That said, that garages attitude is starting to slip, so may well look elsewhere, which ius why I switched to Asda tyres. One glitch with that booking, but Helfrods didn't turn me away... advised of a possible wait, but was still out reasonably close to the slot time under the circumstances.