Subaru Engine Immobilizer Shennanigans

My key for my Subaru Outback no longer remotely locks or opens the vehicle. This has been a minor inconvenience for a while- I have to open the drivers door manually and then operate the central locking to open the rear hatch or other doors. Anyway, today whilst strolling through Newtown I had the brilliant idea to get the battery replaced at one of those little key cutting places that for some reason was located at the back of a tobacconist. Strangely, this is not the only multi-function tobacconist in Newtown, there is another place across he road that features a barber out the back of pipes, bongs and incense blends of dubious legality. Anyway, I digress...

This particular key cutting business seemed to exist to provide employment for the tobacconist proprietors elderly father, who deftly went about opening my car key with a jewellers screwdriver and inscrutably inspected the innards. After much hunting through his poorly organised shelving, he indicated for me to come over and showed me the battery with the number 1620 inscribed on it that apparently he did not have in stock. This is what I assumed, because at this point I realised he had the faintest grasp of the English language. As he fumbled to reassemble my key, I heard the sound of something tiny hitting the floor which I assumed to be a screw, but when he continued assembling the key it seemed the screw was accounted for.

I thanked him and wandered off, half heartedly checking every store that I walked past that looked it might sell batteries for the elusive 1620 with no luck until I finally made it back to my car.

I turned the ignition and the car started then immediately stalled, which was extremely unusual, even more so considering the car had just been serviced. I tried again with the same result, and then my hear sank when I saw the little key light on the dashboard blinking plaintively. It was then I realised that the sound of a tiny thing dropping to the floor as my key was reassembled was actually the sound of my tiny immobiliser chip dropping to the floor and no doubt bouncing off into an awkward and inconvenient place somewhere out the back of the tobacconist/key-cutter business.

Fortunately I was able to grab a spare key, and armed with the working ignition key and my other non-ignition key that was now only useful for opening the drivers door,  I made my way back to the tobacconist on a mission to explain to the man who spoke no English that he had lost an extremely important and expensive thing and could I have a look on his floor? This was accomplished with help from the tobacconist proprietor, and when the old man opened my spare working key we could see a small, black rectangular prism shaped immobiliser chip was indeed missing. After a cursory look on the floor, he gave up and then with a small pair of scissors began fashioning me a replacement immobiliser chip out of a piece of black rubber to fill the sad void in the body of my car key remote. I explained it was a computer chip, which remarkably he seemed to understand, and ceased his miraculous feat of semiconductor engineering with the rubber and the scissors, and we had another proper look.

Finally, only after moving the extremely heavy rickety workbench where he had emasculated my key, the tiny chip was located and the key reassembled. Thankfully, it started my car the first time... but it still doesn't work the remote central locking!

Anyone know where i can find the elusive 1620 battery?