What are the laws about minimum signage and road markings necessary to indicate that parking is prohibited on a road side?

My wife has just been ticketed after parking in a car park off a Leeds suburban railway station car park. Street parking has been allowed there for many years, but Northern Rail have just descended on those who have parked there and given tickets and £50 fines - without warning of a change of circumstance.

Even now, there are no yellow lines and the smallest-ever notices about parking prohibition are located in the car park at the furthest point away from the street that is possible.

What is the regulation and what the minimum notice that should be given of a swinging change to practice?

Northern rail recently announced with pride that over 200,000 passengers a year use this small suburban station, but then assault the pockets of those same station users with these sudden and petty changes. Where is the business rationale in that?