You may have been unlucky with a bit of muck got stuck in the pump impeller, but has now got cleared. Main sign of a pump starting to fail is it not starting to run when CH is switched on.
I had a similar fault some time ago, but the pump has performed faultlessly for about 4mths so you might be the same.
Grundfoss is a good make but you can use any pump you like, provided it has the same ratings and sizes.
It is not recommended to stir round the water in the header tank, some muck does settle in there. So, not a good idea ☺