Have a special magnetic filter fit to your central heating system to catch corrosion debris and sludge which kills boilers for £195.
If you take out the full warranty on a worcester bosch boiler installation. I will fit a magna clean or similar for free.
Your problems may come to an end. Salamander pumps have developed a new pump that can boost your mains flow to 12 litre per min, which means a great shower! just call for a quote to see how you may not need to replace your old combi boiler and just boost the mains water through it.