As far as I'm aware the flight just vanished off Flytecomm at the end of the day. Sorry for the duff gen but Flytecomm is often accurate (but not this time!). I've never heard of an AN124 going into Southampton and I would have thought it would be a bit tight (5400 ft runway?).
Being a gricer (railways),and not conversant with all aircraft types, I am not certain what your Antonov 124 looks like. If you have one I could confirm possibly one has. Several years ago I saw on a couple of occasions, huge (Russian built) transport aircraft at Southampton. In fact their tail section was pretty well as high as the Terminal building, (if not more). At that time their visit was made to collect specific items, however I cannot recollect what they were now. I believe the operators then were possibly based in Belgium, or a neighbouring country. At the time it surprised me such large aircraft could use that airport, especially with the railway works at the northern end, but I can confirm they most certainly did!
An AN124 would not get into SOU, well if it did it would never get out again, i think what stoneleigh would have seen would have been an IL 76 which did make a couple of visits in the 90s helping with oil spill duties
I agree now you mentioned the date, and the purpose for the IL 76 to visit Southampton. Must have been one of the largest aircraft I have ever seen going into there. Pity it was for such a brief period.