Thanks Ian.
I squeezed a bit of bench time in this afternoon to tackle the roll cage.
After a lot of clean up on the roll cage parts, breaking & fixing one in the process, I used blu tack to hold the main hoops in place, so that I could assemble it on the model, rather than separately as the instruction state. I thought that would give me a better chance of getting it all square. I've already done a bit of clean up on the belts here too, and removed one strap to soak in debonder to get rid of the "panic" glue I slapped on it when things weren't going well yesterday.
I wanted to keep the cage removable for now as I wasn't sure how easy it would be to get the driver in afterwards. So I picked up the driver sprue to put him together for a test fit, and found this:
It seems like there's a different, compressed seat to use with the driver, but it isn't mentioned anywhere in the vague instructions. There's no way I'm going to fit it now, so I'll have a go at carving the poor drivers derrière away and see if I can get him to look & fit okay.
Having worked out I'd probably have enough room to get the much slimmer guy in afterwards, I cemented the cage in place (after modifying a piece that again isn't mentioned in the instructions, but I'd noticed in a picture on Gecko's Farcebook page). The cage also has a thin bar behind the seats, which I first broke when cleaning it up, fixed it then lost it , found it, lost it again after my tweezers pinged it into another dimension, finally found it again so nailed it on quickly before it disappeared for a third time. There were also 2 thin vertical PE strips and some mesh to add. This is a bit higher than it should be, but it was fairly square and central at first go, so I decided to leave it alone. The seat belts fasten to the same bar.
I notice in the pictures that some of the cleaned up sprue scars still look a bit rough. They feel perfectly smooth so I think it's just discolouration, but I'll know for sure once it has some primer on.