How to top up pressure on Greenstar 24i

I have done it on a similar model, (25i) you need to take the bottom panel of (pull the handle) and there should be a black hole next to the water inlet pipe, this requires a key to operate which may be sitting in the bottom panel or on top of the boiler, if you do not have it to hand.

 

It will depend if the internal filling link has been fitted or an external filling loop installed.

 

As Joe and Deleted member 11267 said.
Usually a white 'key' that pushes in to form part of the filling loop, then a little white square that's basically a tap to allow the water in.

Had a slightly embarrassing moment once coming to the end of a bathroom refurb. Had drained the system down to finish off piping the towel rad up...about 3pm on a Sat, job done, went to refill and no bloody key! Had to shoot off to a friend who had a bosch boiler to borrow theirs...an hour round trip!
Anyway, I've got one in my tool bag now

 

Been caught out on that one too, I spent three hours looking for my key in the van.....on a Friday afternoon...

 

It is handy to note there are two different types of key.

 

Yep there is...well remembered Deleted member 11267