Sirrus Thermostat ( For Use With New Plastic Piston) SK740012

I don’t think you have done anything wrong it may just need the temperature reset.

http://www.shower-spare-parts.co.uk/uploads/F4295b9d5a146c.pdf

Hi

First check that the supply coming into the shower is hot.

There should be (not always fitted!) filters and check valves/ flow restrictors in the inlet elbows check that they are all clear and working properly.

Also check that the piston is moving freely they can often be sticky they need a good smear with silicon grease.

http://www.showerdoc.com/shower-installation/plumbing-consumables/MCSILICONE-Silicone-Grease-Tub-30ml