Thanks chaps...
On closer inspection, and after more assiduous creosote scraping, I discovered the latch bolts are, as mlappin pointed out, bolts that can indeed be adjusted. So, for now, I have adjusted the bottom latch bolt inwards slightly to improve the seal.
Then, thanks to AirForcePOL, I cleaned off the front off the furnace with LAs Totally Awesome cleaner - "Awesome " is exactly right - then checked the boiler every 12 hrs or so, to see if the leaking stopped - so far it has! Magic!
I also talked to my local dealer about the "Smokin' Swoosh" - she did some research and came up with this response from the oracle... "Yes, I've had mine do that too if it is loaded up with dry "baked" wood and the nozzle is a little choked. The wood gas ignites sporadically and "chuffs". And the baked wood gasifies very quickly and floods the nozzle with wood gas. Usually digging a small hole through the coal bed with poker will send an even steady amount of wood gas / oxygen into the horseshoe brick." Who'd have thunk it?
So, as RSI suggested, this was, in fact, two separate problems and the poor door seal at bottom latch was simply highlighting it.
You guys rock!