All Australian Car seats anchor with an anchor point AND a seatbelt. They MUST be anchored with both for them to be safe.

So with capsules, the base is anchored in and seatbelt goes through the back of them. The capsule itself clicks in and out and is removed from the car for use, the base remains anchored in place.

With car seats (reward/forward facing) the seat itself is anchored in and a seatbelt runs thorough the back.

You can hire capsules all over from various places, usually the ambulance service, but if you contact your hospital they should be able to point you in the direction of the service in your area.

Installing the car seat is really very simple, they all come with instructions. I would suggest you install it in your car before you go over so you know how to do it. You will need to make sure your ILs car has the anchorage hook attached, some older cars only have the hole for the bolt, not the actual bolt installed.
You can also have them fitted at places like the RACV, then they can show you how to do it correctly so you know once you get there. I am sure there must be a similar service in the UK so perhaps your ILs can suss it out for themselves aswell after you send it over.

HTH
Thanks for that info! Its not so much once its there that I'm worried about - its more the travelling to QLD bit that is an issue. I'll be there for a very minimum of a week and the idea of flying into Rockhampton and my father trying to install a carseat in the car park is incredibly scary. And no, chances are they won't hire a capsule for us in advance. Nice huh?

I think they use a different system of securing for child restraints overseas so you may find an Aussie seat won't meet their standards? It's worth checking out in case you can't use yours over there
From the sounds of it our standards are higher than theirs but I'll definitely be checking it out anyway DH is English and I sent him the instructions for the infant carrier that goes with the pram we bought and he called me and said "I'm really blown away by how complicated this is... in England you just strap it in with a seatbelt and away you go!". I know for the toddler seats thats true as i remember my stepson's seat, but I figure he has to have it wrong for the infant ones.