comfrey or borage?

I don't know 

The blue ones on my plot have leaves very much like comfrey so I bung them in the compo bins and my comfrey pipe just as I would comfrey

Borage self-seeds like mad so I have no doubt you haven't seen the last of it Annen 

There are different varieties and it may be that you sowed a dwarf variety.

Borage officinalis (sp?) grows to 3' and I've had a very productive stand of it when sown in mid-summer, so with no compunction I've hacked it down before it seeds.

No, its definitely NOT a dwarf variety, it gets to waist high before it falls over and seeds everywhere, the 20 little plants I took out were just next year's monsters-in-waiting.
CQ, I've already discovered some seedlings I missed!

They come up all over the place. I'd move them to where you want them while they're still small plants as they don't seem to take if you move them when they're big

Borage self seeds with massive abandon - you'll see it popping up everywhere next year if you've had a plant this year. I use the young leaves in salads and freeze the flowers in ice cubes - looks v pretty!

Thanks for the confirmation of variety, JohnB.

I can only put the short height of your borage down to water/nutrition shortage.  It would also explain the (premature) flowering, triggered by the shortage(s).

The self-sown (bless them!) should do better and come up early, as soon as soil temp is good.  I think you're right to re-evaluate this plant, because it's worked well for me as a green manure, albeit a summer-sown one, and is definitely capable of feeding your comfrey factory in a big way.

Tom