There may be internal condensation from the chimney flue that may run down the pipe into the heater.
If it is leaking at the roof join I would suspect the issue is with the sealing there around the external flue outer pipe.

I dont really get that problem as I use the heater with the air fully open until all the wood is burning really well.
Mine only soots up if:
1. If I turn it right down (mainly with hardwood) . Redgum not so bad once its burning all over.
2. If I don't let it do really hot burns.

They need to be run "HOT" A lot of wood is not needed. A few smaller bits will make it glow nicely.
Then add a large piece if needed.

I find the glass residue quite easy to get off.
1.Wet rag or paper towel.
2. Wipe off any loose soot.
3. Dip wet rag into the grey fine ash in the firebox.
4. Rub paste over glass.
5. Wipe clean with a damp rag.

I have only recently had a Nelson Invicta installed in my home - also in Bardon - and am having very similar problems. I would suggest that the problem is not with the heater but the installation of the heater. I bought my Fireplace from Gold Coast Fireplace and Bbq at Bundall. Both they and their
Installer have been completely unprofessional and inept in helping to solve the problem (once they have your money they don't want to know you). Consequently I have contacted Mr Stove in Darra who (even though I had not purchased anything through them) were most helpful and professional and they have recommended a chap in Brisbane who does fireplace maintenance and repairs. It will probably cost me another three or four hundred dollars, but he will be coming out to check that the installation has been done correctly and will make the necessary changes to improve the draught - which I think you will find is the problem.

I was told numerous times the black soot on the glass is due to burning wood that is to fresh/high a moisture content!
Seemed to be justified when varying wood type but still struggled.