It is in TSG Journal 11 (1985) pages 33 - 37; the surveyors were Graham Proudlove and Bob Grimes. We did find toadstone and a bone (mentioned on page 36) so my memory was right after all. It says on the survey that the highest point is a lava boulder choke "100 m above the show cave". This almost certainly means above the show cave floor at the end of the 5 Arches near Buxton Water Sump (remember that the show cave used to extend further than at present).
So assuming that Buxton Water Sump is, say, 3 metres above the Halfway House passage - which is at the same level as the resurgence (approx 190 m asl) then the top of Victoria Aven is at 293 m. The best estimate I can make from the 1:25,000 map of the altitude of the ground roughly above Victoria Aven is 327 m asl. So the top is approx 34 metres below surface.
Hope this helps!