I would fit a liner, and as you say a 904/904 is ideal, needs to be done by a Hetas registered fitter and signed of.
If the problems as bad as you state I would say you are not burning your log burner properly.
Most stoves have an air wash system, which set correctly will burn clean, if your glass is tarring up, then so will your liner.
A lot of people get a stove to large for the room and have to have the air wash controls turned down to avoid burning up to quick, hence the tarring.
Another problem could be that your using unseasoned wood, it needs to be dry.
You should only need to clean chimney once a year, not every month.
I cleaned my after 14 months use and it didnt really need it.
If you fit the liner yourself (and inform bco) make sure the liner is the right way up, ie pointing up as any condensation can run back down.

hello we are a hetas approved company.
first things to check have you a brewer fitted to your chimney pot this is a rain cover damp in your chimney could be createing problem when burning wood.
check moisture content of wood your burning.
It could be that there is not enough draw on your chimney to which there are various methods which can sort the problem.
a liner would be avantagous for correct sized flue , the best thing to do would be to look on hetas website for local engineer and let him have a look.

As above, if the flue requires sweeping once a month there is an issue - also the fact that you are running the flue on an 8" diameter will be drawing more air through the fuel which may well mean you have to throttle the stove back. Fitting a liner would make sure that the stove is running on the correct diameter flue, I would also get a stove thermometer to make sure the unit is running at the recommended temps and a moisture sensor to make sure that everything you burn has a moisture content of <20%. Annual sweeping is the norm however some liner manufacturers and the national association of chimney sweeps recommend sweeping twice per year or in the case of wood up to quarterly when in use.

Martyn Heywood - HETAS engineer.

Don't think you need twin liner