Question was how do i refill it?

Though i have just worked it out while typing this! Presume it must have been low for ages as i had to change a tap over and turned water off etc. I think drained some or a lot of air out of my stupied towel rail. A good old messing around with some levers on the boiler and it has now worked! I like nice and easy fixes lol.

Now i can go and have a shower......