Ash, if you have an optimate try topping up the battery charge, ABS system hates low voltage. Probably not the problem but worth a try.
Depending on ABS system could also be low fluid level in Servo pump, faulty brake switches, lever fouling hand guard, build up of wear particles on rear wheel speed sensor, rear brake light lamp/lamp holder fault, low battery voltage or worst case a fecked ABS unit (£££).
Hope it's something simple, I have a GS911 and I'm in Fareham if you need help.