The HVAC system on my 08MY was a bit smelly, not especially musty, just.... smelly.
Traditionally, the fix is to use the antibacterial spray, put the aircon on full with the recirculation on and leave for some period of time. Having seen these in (in)action in various cars and concluding that all they generally do is kill the bateria in the headlining and give a bit of as chemical smell for a few days / hours, depending on the quality & quantity used, I tried something different.
There is no indication in the WSM of where the recirculation air inlet is on the heater box, best guess is somewhere between the heater box & the bulkhead, I could have smoke tested it to determine exactly but the smoke doesn't smell great. By removing the centre console 'cheeks' the sides of the heater box are exposed, there is a gap against the bulkhead, this is most likely where the recirculated air is drawn in so I put a can of the aircon antibac spray either side, setting the temperature to ambient, fan to ~30% and recirculation to manual 'on', the idea being that as the system works on the 'reheat' principle, this would guide the antibac spray through as much of the heater box as possible whilst pushing it through at a slower rate so as to treat the internals, not the air and cabin surfaces.

Stage two was to treat the heater matrix and the evaporator to a foam bath - not as weird as it sounds, Fortron ( https://fortron.co.uk/shop/aerosols/fresh-zone/ ) and others produce a foaming product to clean and disinfect these parts, usually applied by a long tube down one of the dash vents, which I was going to do until I saw this...

Well spotted on those two, I must admit I have used the anti bac aerosols in the past and read the instructions, ignored them, and put them in the front foot well pointing at the HVAC unit... Foam stuff looks good to....  Pete
I now have permission from the boss to use Y.O.L.O. Enjoy life
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand Sold
2002 L322 Vogue 4.4 V8 Epson Green (Fatty Offroader) Sold
-Click for Project Fatty off roader-

I think the foam did most of the 'heavy lift' in cleaning the heat exchangers and of course the enclosure, the aerosols probably had more effect on the ducting to the vents.
However it worked - it worked.

Did you take the clear plastic cover off of the drain tube & spray up the tube / ducting?.

Its stainless steel, but yes, it looks like plastic in the photo... Its held in place by a T20 screw. I did spray the foam in from both sides but to be honest, you could probably do it from just one side. Make sure you put the cover back on as soon as you finish spraying the foam in so the muck goes out of the drain tube and not on the carpet!

LR TIME on the tube just coverd this using the foaming spray with a long straw down every air vent

The potential problem with that approach is that most HVAC systems default to 'all flaps closed' when shut down, I can't honestly say whether the various iterations of the FFRR follow this scheme or not, getting the foam into the area where I wanted it by removing the caps shown above did get the results I was aiming for however.

Luckily mine doesn’t smell. 2012 TDV8 Vogue (L322)

Good for you. For those who have smelly aircon due to lack of maintenance / use, the above may help.