I don't think they do one small enough to get into the hole - 17mm seems to be the smallest. (Its 9/16" or ~14mm
Anyway, thinking some more, to recut the seat needs to be done with a lot more care - it needs to be square to the centre line of the hole otherwise it will leak even more!
I think Chris's idea is just to remove any build up from the seat and not to actually do any cutting.
So a light spin by hand with the drill bit more than anything else.
Oh and given the PRV valve has a tapered edge to it, I would think the seat needs to be similarly chamfered.