If everything you say is a correct and true record of events (sorry, we're all natural sceptics on here) then it would seem that the sale of the combi to you will be covered as an off-premises contract and as such you have 14 days to cancel. If they didn't inform you of your rights to cancellation, then the 14 days is extended to a year. You don't need to mention this at this stage, keep it in your pocket as a trump card if they get awkward. Getting legal on people too soon just causes animosity
Contact them explaining that you're not happy, set out your reasons why and request that they rectify by restoring your Megaflo to operation and providing you with an equivalent boiler to the one they originally agreed to supply, from a different manufacturer if necessary. Also explain that you won't be paying them until this is done
Contact them explaining that you're not happy, set out your reasons why and request that they rectify by restoring your Megaflo to operation and providing you with an equivalent boiler to the one they originally agreed to supply, from a different manufacturer if necessary. Also explain that you won't be paying them until this is done