I cant help but think that if I'd have done NOTHING then neither would the other aircraft. Particularly the latter one. I'd justify myself as far to say I took control of the situation for my own good, as PIC of the aircraft I was flying. When the second aircraft came on frequency, me asking him to report position benifited the other aircraft I'd already spoken to. I didn't go as far as to tell him about other aircraft I was aware of! But I'm sure G-CD could have spoken to G-BA if he felt the need to.
I wish we had MBZ / CTAF like they do in Oz. That system works so well and doesn't require a ground controller!