Thanks Tony,JS5D, what year is your Rover? Have you tried Roversnorth in Vermont? They stock plenty of parts for the older models. Haven't had many issues over the years with availability from them for my '85 Classic.
It's brakes! Try not to go cheap! I know some OEM parts are pricey, I'd rather have the confidence in my vehicle's braking system than pinching a few dollars.
Sounds like the diaphragm in the brake booster has failed in yours....good luck. If you do change out the booster, install a new steering shaft while you're at it, it's a PITA to change with the booster installed.
Cheers, Tony in Illinois
'96 Disco 1