Hi Michael
My "Brake System Fault" light has been on, more or less permanently since I bought the car two and a half years ago!
The fault is the reverse of yours, it occasionally goes off for ten minutes or so and then comes on again.
When I was at Dave Marks last year I mentioned it to him and he instantly diagnosed it as the "such and such switch" (can't remember the long complicated name for it!)...."it's about £250.00 and is located on the bulkhead". I asked him to identify it for me and on lifting the bonnet, lo and behold, no such switch is fitted to my car
I've now lived with it for so long I barely notice it

Brake fluid level is always on the mark and brake function inc. ABS is fine.
I guess the fluid level sensor is probably at fault but have been too lazy to get involved with it
Best Wishes
DH Dove