The fuel pontoon at Camper and Nicholsons is 24/7 (pompey Harbour, just downstream of Clarence Yard, but you need to check your filter quite regularly if your going to use them!
To use the 24 hour facility you have yo have an account set up and then you self serve on tyhe pontoon by swiping your card.
Ben why dont you just take the fuel to the boat in some jerry cans and use a Fullers Fueller (jiggle syphon) to transfer it, It's a lot cheaper and easier.
I've got one you can borrow if you want to try it out
Here it comes again, I don't stand a chance
Soul possession, Got me in a trance
Pullin' me back to you - Deja Voodoo