There are two to my knowledge one permanent at the little boat yard just up river of the port side and one that has a small mobile wagon. he however doesn`t fill you up until he has checked the others price and charges the same. I guess it will be about 40p ber litre, if last years prices are anything to go by.
Raybar owns both the one on the port side and a small tanker. He prefers that you go to the pump on the port side rather than call him out, in any case he'll only come out for motorboats (150L+). There is now a second tanker that comes round and he is slightly dearer (1-2 per L) and found him to be not as chaep or friendly as Raybar.