HP Laserjet 5000n having problems choosing correct tray

We have an HP Laserjet 5000n which is about 6 years old.
Running Macs on OSX, printing through network.

Suddenly started having a problem with tray selection for A3.
Tray 1 is for manual, envelopes etc
Tray 2 is A4
Tray 3 is A3

When sending a print from, it seems, any program the printer processes the job and then says "TRAY 1 LOAD PLAIN A3"
If you load A3 into the manual tray, it prints fine. If you try and select another tray such as tray 3, which is the correct one, it prints it on A3 from tray 3 but wrongly scaled and positioned too low on the page. It then says "41.3 UNEXPECTED PAPER SIZE"
If you hit 'GO' at this point, it prints it again but still incorrectly!

I have removed both trays and air dusted the whole thing.
I have restarted it.
I have reset all the settings and memory.
I have removed it at the Mac end, re-added it after installing the latest OSX driver and still the same.

Any advice greatly appreciated.