Last night I took the battery out so I plug into a charger to check it had a good charge, showing 80% and took about an hour to get to a full charge

Put the battery back in this afternoon, turned the ignition to postion 2, the PATS led came on for about 3 or 4 seconds then went out, switched the ignition back off then back on again, this time it went back to the previous behaviour of not coming on at all

I did notice that there's a fair amount of corrosion on the connections that the wiring fits into on the plastic module which goes onto the positive battery terminal, so I'm thinking it's probably a good idea to clean all those up to make sure the connections are all good before I start buying parts that may not need replacing

Though why disconnecting the battery would let it work once only every single time I do it and for long enough to drive the car a few hundred yards each time if it is a dodgy connection issue I don't know