I have asked my trimmer to come out and see what she thinks is going on. I previously have been in touch with her through pictures and email. Up to now, neither she nor others I have asked have thought this was a concern, but we were monitoring it to see if it improved, stayed the same or worsened. In my opinion it has worsened in the past 3 weeks and, as I said, I have asked for someone to come out in person for advice. However, thought I'd ask here if anyone has every come across this before.
History is good -- no lameness, was some thrush (hence the heavy trim on the frog) but gone now, no tenderness, no bruising. I can poke around and it doesn't bother her but it worries me that this is so deep.
She has been trimmed since these pics were taken just to do her heels and a roll.