LOTS OF DIFFERNT HEATING PROBLEMS

Please can someone help?
We had a new central heating system installed around 3 years ago (vokera linea 24 combi) and we are getting the following problems -

1.Water pressure is low. Have re pressurised with the filling loop but goes down again after a day or so
2. Some of the rads are cold at the top and some of them are cold at the bottom. One of them is stone cold all the time. I think it is the furthest from the boiler. There is also quite a bit of noise from them.
3. The hot water is hot but comes out very slowly. The cold water is fine.

Would all these be related?

Thanks

sounds like sludge to me

Thanks for the speedy reply

Would it be something to attempt myself?
Not overly hot on DIY but not really stoopid neither.

do a search on "flushing" and see if you feel ok with doing it, its not rocket science but must be done right or you will have problems