Wiring box removal - M73

Heloo guys,

I am in the middle of replacing all under bonnet flexible fuel lines. I've had a splendid couple of hours doing the two lines from the bulkhead. LH side was easy, RH side with the line under the brake master cylinder (my car is LHD) was a right little barsteward but they're both done now. I've also renewed the fuel pressure regulator. I want to renew the fuel lines that link the injector fuel rails but I've run into a problem removing the wiring box. I removed the sound hood, both bars for the hood, disconnected all injectors and a few other electrical connectors, undid the single nut at the back on the LH side. Looking at the box which is now reasonably loose it seems the procedure is to push it backwards slightly so that it disengages from the two slots at the front and then lift it upwards and clear. There seems to be something stopping it moving far enough back but I can't find out what's stopping it. I have looked at the procedure described on e31faq which is for an M73 engine from a 7 series, there are some differences, notably in the wiring box, mine looks a but different with the vacuum lines fitted into the top of the box, not mounted on the side.
Can anyway give me a hint what I'm missing?

Cheers