Thanks for the photo of Lady Leicester and her daughters 992234177.
The tiara worn by Lady Anne Coke, Lady Glenconner is, I suspect, likely to be a Tennant/Glenconner jewel. The family was very wealthy and is likely to have had more than one tiara, but photos of the ladies of the family in grand jewels do not feature much in the press.
Lady Anne, as Lady Anne Tennant, was photographed wearing it at a ball in the 1960s.
The link 992234177 gave to what is described on Pinterest as the Tennant family tiara shows a tiara with large diamond stars.
Lady Anne wore 2 of the stars as brooches for one function.
I have looked in the press but could not locate any photos of the Marchionesses of Abergavenny wearing grand jewels -- so no tiaras. Similarly, I couldn't find images of the wife of the heir wearing a tiara.
The photo agencies don't have many images of the ladies of this family either.
The only image featuring a tiara I found was one on the Topfoto site where the lady behind QEII is identified as the Marchioness. If this is the Marchioness of Abergavenny, she is wearing a different tiara. (I haven't checked to see who accompanied HM to Jamaica in March 1966).
I wonder if Lady Abergavenny, like the Duchess of Grafton, might have borrowed a tiara? Of course, there might be several similar looking tiaras owned by various families.