Not easy to tell from the photo but could be one of the Saxifrage species

With digital photography it is so easy to take a pic of the label as well Sheila?
I'd hazard a guess at a stonecrop.

If you've got that picture on your computer, Sheilabub, use the edit feature to crop the picture and have a look at what you can see, homing in on any of the three labels visible to see if you can read what they say - I'm guessing the one in the middle towards the bottom of the picture might be the one for this particular plant.

Thanks Badfish.
Thanks Urbanite - that's exactly what my OH does, because he's the sensible one (I was on my own that day)! Note to self: 'do that in future'.
Thanks Bamboo - good thinking. Shamefully, I'll have to ask dear OH to crop the pic for me . . . if successful, I'll certainly post the answer.

Could it be Linum flavum? I just Googled "small, yellow rock plant" and one of the photos looked very like this. (I've also sent an email to the Botanic gardens where I saw it.)

Well it does look like it, but unfortunately, your photo is just a bit too far away to see real detail

If you enlarge the image you can just about read the label and I am sure it says Linum.

Thanks Bamboo and Owdb . . . reckon that's it (until further notice!).

Update: I have now heard from Cambridge Botanic Gardens who say that the plant is Golden Flax, sometimes known as Linum flavum.

Thank you everyone for taking an interest!