This afternoon I tried to lock the PLL at end Y.  One problem was that the phase frequency discriminator was not working, I watched Imon on a scope while I changed the laser temperature to move the beat note from above to below the locking frequency, and saw no change in Imon.  I swapped all the cables over to the PFD on the right side of the board, as you are looking at it from the front, which responded as I thought it would.  The chassis has serial number S1000761, I got a spare (S1000764) from the midstation and swapped them, I've brought the broken chassis into the EE shop.  

I don't know what might have happened, we saw that the power supplied to the whole rack was not right earlier in the afternoon, the power that should have been +18V was +25V, which Daniel has now fixed.