I went into the dealer tonight to see what's going on as I'm fed up with the 520d and want to change it. To my surprise I was told that the pump has arrived and they were going to fit it tonight. The bad news was quite interesting, it's not the pump which is holding everything up but a high pressure pipe connected to it.
In short when the pump fails it can increase pressure in the pipe which in turn can fail and is apparently even rarer than the pump.
Anyway they're going to test mine to see if the pipe is OK and if it is I can have it back tomorrow, if not I'll still be waiting.
At least I got rid of the 520d. I've now got a 320d Touring. Life is good