Interesting. Never noticed that before, and I've seen Tony Agar's Mossie a few times.
Info I have normally states that the fairing (apparently without the 'tunnel') is for the 'Monica' antenna, and below this, near the tail light, the infra-red source for the Type F or Type Z I.D. system. Depending on Mark and role, there could also be a 'Rebecca' antenna, forward of the 'Monica' antenna, inside the fuselage.
There are sketch drawings of these arrangements in the book "Aircraft of 100 Group", and although these are specific to those aircraft, as far as I'm aware, the 'Monica' antenna, at least, was standard fit.
In the pic you posted, with enlargement, I think that's the IR source for the Type f or Z I.D. system, but of course I could be wrong.