Very kind of you.
Yes, I had found the Collises in 1881.
I agree any IGI submitted records need to be treated with a great deal of doubt. However, the extracted records that I've checked, I've so far found to be reliable. Still worth getting to the original registers, though, as there is often additional information there.
I'd found that with the IGI Batch no, you can search by surname only and it will find all the children, but haven't tried the method you describe, so thanks for that.
Great Aunt Florence is an enigma. We know which side of the family she was on (my paternal grandfather, so Smith, Stevenson or Burton), but there just isn't a Florence in the right period. The photograph dates to the period September 1877 - 1902, which is when the photographer, Henry Van der Weyde's studio was at 82 Regent Street. Unfortunately my father never asked his father who she was, beyond being Great Aunt Florence, and we didn't push my grandmother hard enough to get beyond the "she went to London: we don't talk about her" - as you say, similar to having a pole dancer or worse today. The moral for anyone out there with living grandparents is "put grannie on the rack till she gives up every last bit of information"
Clearly Florence did go to London, as that's where the photograph is taken. Given the address (on Regent St), van der Weyde was no cheap back-street photographer, so I suspect she can't have been too unsuccessful. It's a very glamorous photograph and I've only just noticed she appears to be wearing a sleeveless dress which sounds a bit risqué for the second half of the 19th century.
Thanks for all your help and pointers. All much appreciated.