I am always unsure as to whether it is worth replacing the foil and cutter rather than just buying a new shaver as the replacement isn't cheap, but it seems such a waste to chuck working equipment and certainly not good for the planet!
In fact each time I have replaced the foil & cutter (about every 18 months) I am always amazed at what a difference it makes; all of a sudden it is so easy and quick to get a good shave again.
The first time I bought spares online I was very unsure that I was buying the right parts for my razor (a Braun 5614), I have now confirmed that when it says "5000/6000 Series" they fit shavers with model numbers beginning 5... , or 6...