Well, well, it seems that being threatened with legal action really does work occasionally, especially when the person being threatens knows they have f&^ked up.
The agency in question have, after several weeks of not contacting me, agreed to pay up over this debacle.
A sum acceptable to all parties has been agreed on.
I’m obviously very happy, but the victory feels hollow, trust and respect for the parties involved has been irrevocably damaged.
Im sure it will feel less hollow when I get my new lens though ;-)