On most Rover's a proper cap is required to maintain fuel tank pressure especially if the car is fuel injected. If the 'all size fits all' doesn't provide an adequate fit then you really should get a genuine cap.
I can vaguely remember my father having problems on an old British Leyland car... He bought an aftermarket petrol cap and it wasn't vented so the pressure build-up ruptured the petrol tank. Best be safe and get a Rover cap.
The one size fits all caps aren't intended for longterm use just temporary. Your best bet would be to get one off a scrapyard it will only be a couple of quid and nowadays they usually have the keys for them.