Get a helper to sit in the car with the engine running in neutral, with the foot brake applied. Then lift the bonnet and give the wire on the right hand side of the brake fluid bottle a wiggle. They are prone to bad connections. Sometimes wrapping the connector in insulation tape does the trick.
Not being funny, but is there fluid up to the max mark in the reservoir?
are there any obvious leaks?
fluid is good and no leaks
Have either of them had owt done with the handbrakes? [Is it over adjusted?]
Make sure the handbrake is fully pushed down. The linkage within the rear brake is prone to binding especially when the weather gets wetter. You may even have to check this out if the problem persists.
have you checked the handbrake switch?