Potterton prima b issues?

hi, we have a potterton prima b heater in our house which we are having a few problems with. The hot water and the heating have gradually faded over the last few months. The costs involved fixing this are our worry so our question is, is this a relatively cheap fix ie a few hundred pounds or a possible replacement heating system change which is out of our budget. I would like a rough idea before calling an engineer so any advice would b helpful.

Thank you