My first thoughts are an oil fire - but note that this is a guess, not a statement of fact, based on the airborne photo, and the latest shot of the close-up of the engine bay.
I might be wrong, but it didn't look too intense, and the 'smoke' seen on the ground shot, with the fire crew in action, is the effect of the AFFF media, which 'billows' and turns smokey-grey on contact. This fire-fighting media is synthetic, and nowhere near as corrosive as the older, bio-based foams.
Also, a photo on BBC local news, showing the aircraft landing, from the starboard side, showed a relatively light trail of smoke, more consistent with burning materials such as hoses or insulation, rather than fuel (for example), with no flame visible.
As for the main spar, PA474 was re-sparred about four years ago, the only Lanc ever to have been so treated and, with the spar some ten feet or more aft of the 'seat' of the fire, and utilising new, heavier gauge material, damage to this area would be unlikely.
That said, it's going to take some time to remove the engine, and strip, examine and replace all the ancliliary systems, as well as the engine bearers etc, which, of course, will be done thoroughly by the BBMF, and I wouldn't be surprised if the Lanc is missing from all of this year's venues.