Back from the vet.
I was aware he'd lost a little weight, but turns out he's lost nearly a quarter of his body weight since his last check-up in November
As he's still lively enough and happy rather than looking ill and scruffy and on his last legs, I opted to have tests done to see whether there's anything else going on with him that can be treated with medication rather than anything invasive.
He's a friendly cat that doesn't get overly stressed at the vet (I mean he doesn't enjoy it, but he's not terrified and will happily cuddle the vet and nurses) so it's not a lot of bother for them to get a sample from him, it's not causing him a lot of distress to have a blood sample taken, just a brief moment of discomfort offset by being bribed with treats.
They estimate about a week to get the results.