1989 Jaguar XJ6 Brake light over-rides the turn signal

I have an '89 XJ6 and I've noticed the following with the left turn signal:

* Left turn signal on and it works and sounds fine
* Apply the brakes and the signal sound starts clicking faster
* The faulty bulb indicator light goes on
* The rear signal is not blinking but the brake light is on
* The front left signal is blinking normally
* Take the brakes off and the signal and sound go back to normal

If I swap in a different rear brake/signal board I get the same results as above

All bulbs and fuses have been checked and are working fine.