Ticketed when a permit was displayed

Firstly, I'd like to say what a fantastic resource this forum is; I spent a good couple of hours last night reading about how to overturn private PCNs. There really is a lot of good information here. Having said that I find myself in a bit of a non-standard situation:

For the past 15 years I've rented a garage from a local housing association. Around the garages there are a dozen or so marked parking bays. When I first rented the garage the parking bays were all numbered and each was allocated to one of the housing association's residential (i.e. non-garage) tenants. The housing association used to gently "police" the parking to help ensure that residents' parking wasn't abused by people doing a bit of shopping locally.

Roughly 12 years ago the housing association engaged UKPC to "manage" their parking. I received a letter explaing the situation and saying that only permit-holders would be allowed to park in the marked bays. Since parking was limited, permits would only be issued to tenants of housing association houses. At the time that all seemed fair enough.

However my garage is right at the end of a cul-de-sac. There is plenty of room to park a car outside my garage without blocking access to any of the parking bays or any other garage. I therefore contacted the housing association asking if I might have a permit specifically to park in front of my own garage door. Nice people that they are, they could see my point and duly sent me a permit on the understaning that I could not use any of the marked bays. In the section where the parking-bay-number would have been written they put my garage number. For the past dozen years I have been able to, and indeed sometimes have, parked my car outside my garage, usually for short periods while I nip indoors for something or drop something off.

Fast-forward 12 years and UKPC are still "managing" the parking around the garages. There are still marked parking bays, though they are no longer numbered. My car was parked outside my garage with the bumper within a couple of inches of the door. The car certainly wasn't blocking access to any other garage or to any of the bays. My UKPC parking permit was clearly visible in the windscreen. Despite all that, a UKPC employee issued a PCN on the grounds that I was parked outside the marked bays.

There are at least 3 standard UKPC signs around the garages though the sign on a lamp-post nearest my garage has been vandalised so many times that it has now been fixed above head-height so it's not easy for a varifocal-wearer to read!

So to the big question: what's my best course of action to overturn this PCN? I'm thinking:

1) Talk to the housing association. Unfortunately I don't think I have anything in writing from them regarding parking; just the parking permit itself. All I can hope is that they have kept a note on-file, but after 12 years that's a bit of a stretch...
2) Wait for a few weeks (as advised on the newbies thread) and then use the PCN appeals process. My grounds for appeal would be that my designated parking bay is outside my garage, my garage is numbered (marked) and that this is noted on my parking permit. But I'm not sure that an organisation well-used to rejecting appeals would have much trouble rejecting that argument, especially if I can't provide any support in writing.
3) Start down the route outlined in the standard letters in the newbies thread. Having said that the UKPC signage around the site is reasonably new (so I'd expect it to say all the "right" things) and, with the exception of the sign nearest my garage, seems quite legible.

Or in this case is there a better/easier/more-certain course to take? I'd really appreciate any advice that anyone here can give me. I really don't like the idea of paying any money to the likes of UKPC, especially when my car was parked in an agreed and acceptable manner!

I realise that's all rather a long story - many thanks for reading to the end!