No masking required although I was initially going to mask it. Just got to be carefull, don't keep the gun concentrated on one area for too long. I used the flat narrow tip for the gun so more controlable where the heat goes, and had the gun around 10cm's away from the panel. Basically start at one end, pointing the heat gun at the one section for no more than 2 to 5 seconds then move along the part slowly, so it's spending 2 to 5 seconds heating the bit that's in front of it. I had the tip horizontal, so it was only really heating the plastic, not the paint (although some heat obviously transfers as it's blown over the plastic). I moved the gun slightly up and down towards the edges of the trim as I moved along it. I did 3 passes over the trim like this, then stopped to let the trim cool down and also let the gun cool down. Once gun has cooled down, move to next bit of trim. Once all are done, go back to the start and do it again. Think in total I did 9 passes over the trim (with the stops as mentioned above) and it came out really well. Like I say, just got to be careful and methodical with the movement.
And your spot on mate, the theory from what I've read/watched is that the natural oils and dyes in the plastic move from within it back to the surface, which leaves it looking like new again. Highly recommend it! Going to give DD's one final pass, then might do poppy's arches/kit (although it's still pretty black tbh).