Yes, as she went back in the Museum in 1988 - I have a photo of her arriving at York from Marylebone in lovely evening light.
Unfortunately Six Bells doesn't seem to have those trips, which were done in Spring - daffs out but trees not in leaf. I might have to rescan a couple at a higher resolution to be able to read the headboards, but Mallard went off towards Harrogate & Sir Nigel came from the west, from (I imagine) the S&C.
Also unfortunately the film was bodged when developed and was cut in the wrong place for one frame, and there is a little stub with the last frame on. However I have a holder on it's way that should fix that problem - it should be able to hold the two ends close enough to scan the affected frames. One of those is Mallard 'somewhere' with a 'Scarborough Flyer' headboard
Ah - but possibly 25th April 1987.
https://www.flickr.com/photos/104413683@N07/20442817918/
Nothing on Six Bells for that, but I would be amazed if it it's complete. It would be nice to be able to search by loco!
OK after plenty of Googling I'm happy with 25th or 26th April for the photos. The headplate on Mallard is the Scarborough Flyer which seems to be a special run by friends of the NRM, and only they used that name. I can't find enough Sir Nigel (so far) on the 'Pennine Limited' to pin down the date, but I have found a photo taken at Appleby that year. I have however found a record of it pulling the Pennine Pullman on the 25th April. Strange! So I'll settle for Saturday 25th April 1987.
Here are two of the photos, colour isn't great and it was an overcast day hence the haziness.
OK the final word. It was the 25th April:
settlecarlislesteam.co.uk