Sarah Siddons is indeed the one I was thinking of!
I saw it running on the Amersham line on Sunday - I remember it (a bit hazily, since I was only 9) at Steam on the Met too. It's a rather sweet looking thing!
87015, is it definitely not passed for NR? I'm not really sure, but I did read somewhere that it had been modified so it could run on 3rd rail only as well as underground lines - I'd have thought this was to give it more lines to run on, but have no confirmation!