Many thanks Adam for your speedy reply. Sad news for me, "Oh no" is my (publishable) thought.
However, perhaps there is one thing that may save my day:
Although it now appears that the original, signed, updated lease is probably lost, I do hold an electronic copy (PDF) of this signed original.
Please can you advise:
a) Will the Land Registry accept this electronic copy of the original (printout or PDF file) for the purposes of registering the updated lease?
b) If so, am I allowed to send this electronic copy to the Land Registry myself, or must I engage another solicitor to do this on my behalf?
Thanks