@ Irish - OK, bear with me, 'cos I am testing my memory here, but my tired brain cells say;
There are two square pins. One is moved when the barrel turns & allows the whole lock & handle to turn when it is central, & the other holds the barrel in place to stop it falling out of the handle. You need to tap out the holding one. If you tap the wrong one out you shear off a tiny round lug from the bottom of the barrel & then it's a real pain to try to fix it. If you do that then it's probably best to get a new barrel. When I did exactly that, I drilled out under the lug, tapped the hole & put a tiny threaded rod in there to replace the lug. I never want to have to do that again!
Once you get the barrel out, you can remove something (Probably a circlip or two) to be able to get the pins out of the slots which hold them. The pins are flat spring-loaded things. You can shuffle them around to hopefully fit the key you have, as I don't think there were too many variations. If you can't fit the key, then just put in the pins that do fit the key, & leave out the rest. It will then be easier to pick, but it will still work as normal & you won't be able to tell from the outside. It will work normally with even just one pin.
I hope this makes sense to you! Good luck!