So, I wanted to round up the conclusion to this.
The park brake fault was caused by a defective switch. More specifically the switch LED was intermittent (the one that indicates that the brake is on) and if the car's ECU notices this it flags up a fault. Apparently this is a common problem. It doesn't stop the park brake from working though.
I replaced the switch - cost about £18 and took about an hour. You have to remove the centre console which is a bit of a job.
Interestingly enough I took the old switch apart recently to investigate why the switch had failed and to my surprise I found evidence of water damage on a pair of resistors that feed that indicator LED. It looks like it may have been caused by a spilled drink or something of that sort, I know I have never spilled one so must put it down to the prior owner of my car.
Something to watch out for. If you do spill anything on a switch like this, then clean it and dry it immediately.