Condensation/Mould Solutions - PIV

WIAWSNB

Please keep the updates going.
It'll be very hard to tell the difference in energy costs in a short time, because every day is different, and the temp - for example - has dropped significantly for most of us recently. It'll likely require an all-year energy kWhr cost comparison.
And to sense whether it's making the house colder will also be hard to do, because if the CH is running, then it'll just run a bit harder to cope; the overall 'feel' will be the same, but clearly more energy will be used since you are now flushing out more of the house's air, and that includes heat.
The northeast corner room has two external walls - that's significant, as well as it not benefitting from much solar gain. What size rad does it have, and does it become 'hot' when required?
What room is this - a bedroom? Is it being used?
Could you cope with the disruption of having the two ext walls internally lined? If so, and this is a valuable, used room, then that should be transformational - say 1-2" insulated plasterboard bonded on. 
Are your floors suspended timber? If so, ensure they are draught provided.
Anyhoo, sounds good so far