Fitting a shower bar mixer unit

[churcheto]

Good Morning,
I hope someone can help.
I have a shower mixer unit which is fed by a gravity system. The water flow has been fine, but recently is not always great and my plumbing merchant said it was related to the thermostat. I purchased a new one and assumed it was a case of taking the old one off and fitting a new one. These seems not to be the case at the water flow is worse than it water before.
The unit I took off has what looked like little ball valves in the water inlet which I could press in with very little pressure. The new one has a metal mesh over the hot and cold water inlets and a valve looks to be behind this metal mesh. I also noticed that the mesh looks like it is held in place and maybe able to be removed with some form of hex/nut shapes tool.
I am afraid I do not know the brand of this as it came in a white box and the only instructions with it was for fitting the shower riser
Do these units come with high pressure washers that need to be removed on some models?
Any help would be appreciated

 

[Plum Bob 66]

Attach an image of the valve if you can and that might give us a better idea of what type you are talking about make/model ect
Bob