No charge from the nice WB engineer.
My original CH installers were the installers from hell. A verbal quote of £3K for a basic CH system in a 3-bed semi ended up costing me £7K.
As for no confidence in original CH installers:
No room/house sizing, no BTU calculations to size the rads, no Powerflush even though I was charged for this, five weeks of further visits at the end of the job to rectify other issues, couldn't diagnose blocked filters in newly-installed shower - costing me £1,100 to get a new water mains poly-pipe as they said it would fix the pressure issue. I then paid a proper plumber a further £120 to clean the filters in the new shower as the original CH company didn't flush the pipework.
The workers would just step into my bath with their heavy work shoes and scratched it up badly. The pipework was all experimental and they eventually capped off a lot of their pipework when they couldn't reach under the kitchen cupboards properly. This cost me a further £500 to have a proper plumber reduce the pipework by eliminating all the redundant pipework.
Each room had around 2-3m of trashed floorboards after they left.
The boiler wasn't commissioned properly. And now the WB engineer has stated that my mains gas flow rate is below the minimum rate required for the WB 30CDi boiler.
And finally I have a leak in the CH system - this is going to cost me £350 to fix.