Ok have stripped it down (don't turn it on, take it apart!) and found no major crimes other than the encoder knob being bodged pretty badly and the fan being duff. No sign of any repairs on the output side of things which is a common failure. Keysight still stock the knobs so I will buy a couple of them I think for stock.
It has the panel cutouts for the TCXO option and the board connector (SMB by the looks).
Gubbins:
Rear panel holes:
Someone is selling original sealed manuals on ebay so I might grab that as well. I'm sure there's a TEA term for obsessive completeness.
Edit: damn I love HP / Agilent / Keysight. Phoned them up, answered by human straight away, took order for parts over phone.
I bought two knobs (incase I munt the replacement in the future) and they were £2.28 each