Shower mixer handle keeps dropping

Do I need to replace the cartridge or is there a fix for this? When I remove the handle the plastic lever doesn't drop. So something presumably has worn inside to the point where the heavy handle pulls the lever down.

See video:

Once the water heats up, it stops doing it.

Should I replace the cartridge or is there a fix for this?

Take the handle off and see what's actually moving, probably the cartridge getting loose though.

Can't get that backnut off. It measures 30mm across the flats, which according to this chart, is a 1/2in bsp back nut. So I tried my 1/2in box spanner (which measures 27mm across flats). I feel like I've been here before when I was trying to take off a backnut from some bathtaps. What the hell is this size? It must be standard, especially on the back of taps. The other end of my box spanner is 3/4in and measures 32mm, but problem is, it won't fit in the chrome shroud. I could use waterpump pliers, but it's a bit heath robinson. Cheers