HELP No water pressure with Boilermate 3

Hi,
we moved to this house 3 years ago.. that is when the long journey began...
To make it a short story.. we have a BoilerMate 3 connected to a Potterman.. our problem is that we dont have much water pressure.. but the hot water doesnt have "any" pressure...(the cold is bad, but the hot is even worst)
We have changed the exchange heater 2yr ago as we have been told it might be that.
We have had at least 3 guys come to see what can it be.. the latest one said we need to change the exchange heater again

I am desperate for help..

Any suggestions on how can we increase the hot water pressure? Is it possible to add a pump just before the shower...?
Have any of you had similar problems with water pressure...?

Thank you very much in advance for your help

You need good cold pressure.
Check the water hardness for your area.
Local water web site should tell you.
If it is really bad.
Then the only answer is a water softner.
THere are some parts of the UK where the PHE's will scale after 12 months!
Mixer valve could need changing too.
PHE can be descaled.
Mike