This is a very useful guide.
I work in a store which has paypoint and, having joined giff-gaff a few days ago and while waiting for my sim to arrive, I checked out the paypoint options to see if giff-gaff was there.
It wasn't, as I kind of knew, but today I did a bit of hunting and found the "mobile and overseas," top up option etc. and managed to find the giff-gaff top up option very easily.
The thing is, I wonder how many stores with paypoint are even aware that this can be done.
Certainly none of my members of staff were aware.
They are now!