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WAIT a second here. Right near the end of @dandub1960 's 11:04 comment above I read "distribution system is two pipe stream" .

That's fine, but makes all the comments above about mod/cons and so on quite irrelevant.

Now. Let's get back to some basics. Our OP's complaint appears to be that the house is draughty. Quite possible, given the age and the mention of metal framed windows — which were junk when they were installed and haven't improved with time.

There seems to be an impression that the problem is the oversize chimney. That probably isn't helping much, but is unlikely to be the major problem.

Assuming it is a significant problem, though, the only approach which will do much to address that problem is to restrict the chimney itself. Since there is a draught hood on the boiler, messing with the boiler air supply isn't going to address the problem, unless the boiler and chimney were placed in a sealed room. Difficult to do, but not impossible. Then you could add an outdoor air supply to that otherwise sealed room and let it go.

A cheaper approach will be to insert an appropriate sized liner in the flue, connected to the boiler draught hood, and seal between that and the flue (airtight seal at the bottom, screened at the top , or alternatively fill between the liner and the existing flue. Which I don't really recommend…

Again, forget about the equipment until you fix the thermal envelope of the structure. If you do, it will take far less boiler to do the job. If you just throw more equipment at the problem the utility will thank you for wasting so much energy and paying for it. You don't have to rebuild the house but close the barn doors then think about the boiler. BTW, the standard is to have a blower door test done with infrared thermography, which will generate numbers on just how leaky the house is as a baseline. Once you tighten it up, you can re-test, calculate the improvement, which can then be used to calculate the savings and if further improvement would pay for itself. You can install a TurboBlaster 2000 with the flux capacitor and inverter raterfrazzer but if the building is a sieve you're wasting money.