Has anyone used the B and Q "Premium Door Buffers" and are they any
good?

I have used B&Q buffers on my MFI Hygena kitchen. They are excellent.

Also could anyone tell me how easy they are to fit to a kitchen after
it has been installed?

On the cupboard doors, they fitted simply by screwing into existing unused
hinge holes without further drilling. Clearly kitchen carcasses are
sufficiently standardised to allow this.

On the drawers, I drilled my own holes into the side of the carcass using a
5mm bit (from memory - check size). This was easy and effective. It was also
less intrusive than following the instructions and drilling into the top of
the carcass, where it would be more visible and interfere with the drawer
contents during closure/opening.

Christian.