Shower mixer handle keeps dropping

[JT101]

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?

 

[denso13]

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

 

[JT101]

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