CL's pricing themselves as sites?

"I just searched for one around Alnwick as I needed a water topup, and thought a cheap CL would be the ideal place to do that ..."

" ...are there any other options to simply get fresh water fill?"

This seems to be what the OP wants, no indication he wants to stay or pay the full fee. Of course, if Gary would like to come back and confirm that he is looking for an overnight stay at whatever fee the CL charges, including the ability to fill with fresh water that would put a different complexion on things. 

I would say just call at a single house or farm out in the countryside and ask nicely if one can have water. 

An Italian couple in a campervan called here one day with that request on their way up the lane to overnight on the edge of the Moor. Nice people and we had a chat after  they filled a 20 litre can. They even offered to pay but I told them not to worry. 

Well NellietheHooker is from Cumberland and I'm sure, in fact almost certain, he has been to one or two CLs outside of Cumberland/Cumbria.  People from that area are allowed to travel outside of it.

......"Except I can't find a single one under £15" which suggests that, if the price were more favourable, he'd be staying?

he'd hardly be considering paying £15 for just a tank of water, would he?

as I needed a water topup, and thought a cheap CL would be the ideal place to do that ... to simply get fresh water fill

I can't see any indication in the OP that he would be staying at all. All it said was a water top-up and a CL would be the ideal place. I think the simply get gives that.

Perhaps the OP will come back and clarify?

Not sure how you interpret that, BB, there is absolutely no indication that Gary wants to stay on site, whatever the price. He has said twice that he just wants to fill up with water. Couldn't be clearer as far as I can see! 

But please do carry on "reading between the lines"!  

I get it now