Can anyone recommend: Gas fitter

You first of all need to determine if you have a good enough water pressure to reach the upstairs taps.  If you have cold taps that work well upstairs then pressure is not the problem.

Something is restricting flow to the upstairs. Are there any taps that may not be fully open or is any pipework damaged (flattened or kinked)? If you have any braided stainless flexibles in the system check them for damage or twisting. A lot of people fit these wrong and wind them up or bend them too tight.

Make sure none of the outlets are limed up.  Remove those pesky flow straighteners on the tap mouths.

If the water heater is feeding the downstairs taps OK then it may not be the heater.

Thanks for the info. It,s not the cold water pressure upstairs thats a problem it,s the hot water.

Just had new shower room fitted and pipes checked for blockages and new mixer tap installed.

the problem is the hot water pressure from the boiler, it has no pump installed therefore relies on mains water to overcome the gravity of about 3 mtrs upstairs.

Looking at fitting a pump but would it be best on the inlet or outlet of the boiler?

I'll try to offer more advice, so here goes.

I have a Junkers boiler and there is a small filter on the side of the water part of the control valve near the inlet pipe. Not knowing  what model yours is makes this is a bit tricky.  It's possible it actually may be a small gauze filter IN the inlet pipe. Mine is very fine so could become blocked if the incoming water has debris in it.

Please send me a PM with the model number and I'll see what I can find.

Mine runs on a pump from a tank in the garage and supplies the house at a pressure of 4 bar (56psi in proper money), it cuts in when you open a tap.  I have one tap in the kitchen on the mains for drinking water, but I am still here having drunk water from other taps!! 

That pressure should give you a good strong shower, but you will be limited by the size of your water heater, an 11lit/min  should be OK.

It's a fact of life that the pressure in any closed hydraulic system is the same. So I still think it's a flow issue not pressure, if the cold taps work OK upstairs.

I hope this helps