You don't have to pay anything at Pultney. If you pay you can use the facilities at the leisure centre.
Best place to moor is on the K&A top of the locks, it is a 10 min walk from there to the Abbey. It's a real pain going down the locks onto the river.
It is very rare to have ropes cut, most times if at all, the ropes are undone not cut. I was cruising the K&A for 10 years and only twice had my ropes undone.
I never came adrift as I always moor up with front, center and back ropes. It takes a lot of time for a yob to undo all the lines, plus I make sure he/she will have to get on the boat to do it.