Sussed it! In between the T and 6 keys there's a little plastic tab that needs lifting up, then you can slide the whole bottom metal plate off.

I traced the fault to the membrane that goes up to the connector - a couple of the conductive traces pretty much rubbed off 🙁 Just ordered a silver conductive paint pen to see if I can repair this. I've discovered touching the membrane even for a moment with a soldering iron REALLY doesn't work!

Cheers,
Steve

Managed to fix it 😀

I bought a silver conductive paint pen and tried repairs and it helped one set of buttons but seemed to generally make it worse. I ended up snipping off the end 3/4cm from the ribbon then scraping away the plastic on the conductors, then put a bit of insulating tape on the bottom to 'thicken' the bit that goes into the connector and now it works!