It's clear that your car was protruding into the footway, (the garage entry is a crossover over the footway). This will be the reason for the PCN.
Looking back on the GSV views over previous years there aren't many when the garage is close, but this one of March 2018 backs up your narrative of parking there being tolerated, and all the vehicles are blocking the footway ! So it does look as if either (1) somebody has complained, or (2) there is "new broom" in the parking enforcement department.
I have to reluctantly say that the contravention seems to be made out, so the PCN is correctly issued. However, can you please post-up the back of the PCN so we can see if the "small print" contains fatal errors.
In terms of a defence, you say people have parked in this way here for about 20 years without getting ticketed, so you have a potential argument of "legitimate expectation" of no enforcement. If you want to follow this line, you would need to assemble some evidence of non-enforcement as back-up.
Anyway, wait a bit for others to comment, but don't wait so long you miss the deadline to pay or submit reps.
