My shower is running cold and my hand control set is stuckl The unit is 811146 Sceptre Universal Thermostatic mixer shower

CarolBeddall, January 2020

Sounds like you will need to buy and fit a replacement mixer cartridge that's within the shower valve body. I think that your shower was made by Bristan and re-branded to be sold by City Plumbing Supplies, so you may need to contact them first for spare parts.

Rupert, January 2020