Luke....let me know how you got on....I got flashed by the same gantry last night in very similar circumstances. Sped up to get past a gumpf doing under NSL in the 3rd lane with no-one in lanes 1-2 or 4. Instead of curising past at a reasonable speed using an illegal undertake I sped up to make an overtake in the 4th lane and flash flash!
The camera is on the anti-clockwise side of one of the gantry's between J18 and J17.
Squeaky bum time for the next 2 weeks!