Being a bit thick here, but I have a grease gun similar to this:
http://tinyurl.com/bvjee69 (or
http://www.screwfix.com/product/jsp/... &docRoot=http
//s7g3.scene7.com/is/image/&viewerRoot=http://s7g3.scene7.co
/is-viewers-4.1/&company=ae235)
....which I've been trying to re-load. I use it very seldom, and think
I've only ever actually charged it with grease the once, when I bought
it; and ISTR did so using a can of the grease which you had to push the
base of the gun into, against a metal disc with a hole, 'floating' on
the grease?

This time I've bought a cartridge of the stuff, and I'm obviously being
a bit thick cos I can't figure out how to get the grease into the gun!
Either that, or something's maybe broken...?

The cartridge has a rip-off tab at one end (which I've duly ripped off);
and the other end has a removable cap. The grease gun has a
syringe-like plunger at the back, with a small toggle affair (like the
one at http://tinyurl.com/bvjee69) which doesn't appear to be doing
much. I've tried variously shoving the cartridge into the gun (won't
go) and pulling and pushing on the plunger all to no avail. How's it
supposed to work? Does the cartridge stay within the grease-gun, or is
the aim to eject the grease from it, into the gun?

Thanks!
David