A few months later I've now been away and come back.
This post on a different thread shows the results of using the 320mm zoom on our holiday in various of the Shetland Islands, and I'm extremely happy with them. Some, but not all, of those photos have been cropped, and all have been shrunk down to a lower file size and slightly lower JPEG quality. I haven't used any image manipulation software at all to adjust the exposures, but I can see how doing so would help some of them.
Since the box is sitting in front of me, what I've been using is an FA80 (f4.5) - 320 (f5.8) zoom. The interwebs tell me it was manufactured between 1997 and 2002. The interwebs also tell me that it's useless for birds. I think I might have demonstrated the interwebs wrong. We did also see seals and (just) an otter, but they were far too far away to take a photo of. I'll have to go back and try and find an otter, but I've already got plenty of seal photos from Orkney.
I still have
@User's 500mm lens, which I will return. Thanks again to all for your suggestions.