Can anyone tell me which unit this is, please, as I need to replace it (not shutting off reliably). I'm thinking Gerberit but it appears to be a 3/6 litre and I'm struggling to identify.

That is a Macdee dual-flush valve, the white body with the blue internals and the numbered height marks on the side. The good news is you do not have to replace the whole thing. A valve that will not shut off reliably is almost always a worn or dirty flush seal, and the seal washers are sold as spares. Twist the top half out of the basket, clean the seat, fit a new seal and put it back on the same height mark.

7 hours ago, stevenlee said:

Twist the top half out of the basket, clean the seat

Agree with cleaning the seal, but on the three I've done you can put it straight back in without a new one and it'll hold for years.