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
I can answer this question...
How can I fix my shower.Sceptre universal thermostatic mixer shower?
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