Your ficus needs as much direct light taking on its leaves as you can possibly give it. A south or SW facing window is recommended, with no curtains to block the sunlight. Outdoors is best for the summer.

The stem will thicken enough to support the plant IF it gets plenty of light, so keep the support until it can stand on its own. If light just isn't available you can just cut it back until the stem keeps it upright.

What do you mean by cardboard?

Cardboard like the box it came in

It just so happens that a quote from one of Al's posts is on my clipboard because someone else just asked the same thing. The first time you repot, "saw the bottom 1/3 of the root mass off, then correct the problem roots (encircling, girdling, j-hooked roots, roots growing back toward the center of the root mass, and roots growing upward. Next repot, remove any roots growing straight or almost straight downward. The ideal situation is to have a fairly flat system of large roots near the top of the pot radiating horizontally from the trunk, and lots of increasingly smaller roots radiating off the larger roots near the surface that serve as anchors and plumbing."

All that being said, does it need repotting? What kind of soil is it in? Does it seem rootbound--if you de-pot it, does the rootball come out in a solid mass? Or do you just want to get it in a better soil (I can't tell what kind of soil it is from the picture)? The pot size looks fine to me, especially if it's in typical potting soil that retains water (which kills roots by drowning them unless you're careful with your watering regime) and will stay even wetter if the plant isn't big enough to populate the whole thing with roots.

The other thing you need to do is read the other FLF posts on here so you can see some of the common problems others are running into and hopefully avoid them by incorporating good cultural practices. There's certainly no shortage of reading material on this particular plant :)

Litterbuggy, thank you for the info on root trimming :) I don't think the rootball is too bad?

The soil has some bark near the top and some (what looks to be) pearlite mixed in, but I think it's mostly compost. I'd like to put it in a heavier pot (but am happy to place inside still in it's original pot) and I'd also like to change to the 5:1:1 soil mix

Liz, I've read a lot of the FLF posts already, just couldn't find anything really conclusive about the small brown spots, one said to change to the 5:1:1 mix as I'm intending to do but I didn't see an update as to whether it worked!!

Does anyone know how much garden lime you need to put in the 5:1:1 mix and if it's necessary?

That's really helpful, thank you Al. I'm keen to get it into a heavier pot to make it a bit more stable, so I will repot into a pot the same size with the 5:1:1 mix, it'll ease my mind about the soggy soil!! I'm in the UK so hopefully it won't be a problem to do this now?

How do you ensure you have got the new soil around the roots sufficiently?

Re-potted with 5:1:1 mix, roots cleaned/trimmed (careful to keep them wet the whole time), everything is crossed now!! I watered til it came out the bottom, at what point do I want to water again, do I want to wait until it's completely dry (checking with a chopstick) to water again?