I have an outside wall painted white which is now starting to crumble and peel badly. what's the best way to fix this?

Scrape away all loose paint, if you have bare brick or render that feels powdery it will need stabilizing solution to seal it, then repaint the wall in masonary paint... do this in drier weather - spring time to give the wall time to dry out if damp has got in.

TC Property is correct, i would certainly be worried if you had the wall painted in the last 3/4 years minimum, after this time and depending on the position of the wall you could just say it`s weathered badly.

Firstly I would find the reason why the wall is crumbling and peeling, could be a more serious damp problem.

sounds like the actual render/brickwork has had many damp problems 1 to many and thats why the paint is flaking etc.

I would check to see if there is damp coming from inside the wall. If not use an oil based primer prior to painting