Difficult to give advice without seeing job but If gas pipe runs ok, I would be biting his hand off at that price, if inclusive of vat as it says.
You already have a baxi 105HE so WHY not have a baxi duotec 28 He A in place it fits onto existing jig with no pipework alterations at all and flue hole only needs a slight alteration .
Labour saving as old boiler can be off and new on and back on and working very very quickly.
Also it comes with a SEVEN year no quibble g`tee.
WHY are you changing a 105HE these are a good no nonsense boiler that are probably the easiest combi out there to fix ???
Most on your list are radiator and circulation/balancing issues which will apply equally to a new boiler.
You have a leak and a noise from your boiler! That's all!
Mostly I can fix leaks within my diagnostic fee and even if a part was needed its usually only round about £20.
The noise is likely to be cured by a proper service which would also be included in my diagnostic fee.
Unfortunately, because householders get seriously worried by water leaks they panic and are easily persuaded to have a new boiler fitted. Most water leaks are very minor repairs. HEs don't want to do a repair if they can get the bigger job of fitting a new boiler.
The increase in efficiency of a new boiler is only likely to be 6% to 12% which does not justify the cost of a new boiler.
Tony
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