Thanks All, I nearly trimmed it this afternoon, but wanted a second opinion before getting carried away with the footshears! Is this Shelly Hoof then, or something else? I couldn't find any decent photos of it online.
Spray Engemycin, inject alamycin (if limping) but don't trim.
That's good advice for footrot, but unless I'm wrong, this lameness is a physical ailment due to getting dirt and stones impacted under the outer hoof, rather than a bacterial infection. I've noticed no inflammation, heat or smell, so I'm not sure what antibiotics would achieve here?
Katrina - I just scrubbed it with a hoofbrush and water (Asda sell them for a quid each, but they call them tile and grout brushes for some reason

), then used one blade of the footshears to dig out the mud. The cavity on this hoof was so open that it was very easy to clean out. The other foot wasn't nearly so straightforward.
I'll have a go at trimming tomorrow, and will try for some before and after photos as suggested. Bear in mind that I'm a beginner with sheep myself though, so the results may not be perfect!
If you can add some seaweed to their concentrate, or offer as a lick that may help.
Do you know which particular minerals the seaweed is providing Anke? I've been thinking of getting some mineral only licks, as the only ones I've got at the moment are the molassessed high energy buckets, so I wasn't planning on putting them out until about 6 weeks before lambing (is that correct?).