GoSmoke - Might I suggest that you copy your post to David Cockburn at the CAA? He is always interested in such safety related matters for his flight safety road shows.
tuniphead, D129 is active H24. Whether that's with military parachuting 100% of the time, or military parachuting 20%, gliding 80% doesn't matter. Unless you received DAAIS from Brize Radar stating that it is not active, you must assume that it is.
Illegal formation is indeed very dangerous; however, to get close enough to an infringer to note type of a/c, colour etc may be sufficient if corroborated with the relevant ATC radar tapes to track the offender to point of arrival. The CAA's enforcement branch take a very dim view of Danger Area infringements.