No luck so far - I'll keep looking (hope I didn't imagine it).
Meanwhile, this blog post by Mr Mustard offers encouragement
Is this a valid yellow box junction? Needless to say Enfield Council are still issuing PCNs here, at the supposed junction of Southbury ...
There is also a key case on the London Tribunals website which sets out very clearly that box junction markings which extend beyond a road junction itself are not valid to enforce, and this seems to me to be analogous to your situation.
Extract as follows:
"The problem with the special authorisation is this. Part of the box that it purportedly authorises is not within the junction of the roads. At its southern side it includes a triangle of road that is not within the junction and is therefore not 'at a junction between two or more roads'. The prohibition, however, relates only to a box junction as defined. The special authorisation therefore purports to extend the prohibition outside the junction. It does not seem to me that the power to authorise signs empowers the Secretary of State to effectively redefine the prohibition prescribed by Schedule 19.
This is not an insignificant matter. The box as purportedly authorised extends for a significant distance beyond the junction itself. This increases the distance the motorist has to traverse without stopping, and therefore makes it more difficult for them to make progress without falling foul of the box. It is also difficult to see what the point of extending the box beyond the junction is, given that the purpose of the prohibition plainly is to stop stationary vehicles blocking the junction to crossing traffic. Clearly, stopping in the area beyond the junction would not block the junction.
I accordingly find that this yellow box is not a lawful road marking. Therefore the contravention did not occur. I allow this appeal."