So, I know that this is not an original idea, but for anyone thinking about boosting their SIII fan throughput, I have found that using -
Marine In Line Blower 12V 3" Bilge Head Galley Engine Ventilation Cooling Fan from eBay
Was very successful.
I mounted it between the air feed pipe to the matrix and the matrix itself (it pushes straight in and the feed pipe goes straight on) and wired it using a relay and fused spur.
Now, I have a constant whooooo from the fan, rather than an occasional pfff.
I haven't had to use it in frosty anger, yet, but it's bound to be better than before.
Regards
Michael Stephen