I think both cables need to be replaced although the high loss of the LO cable has, for now, been overcome by adding a PGA Amp in the LO box in the shack. That delivers +20dBm to the RG58 coax and +9dBm to the transverter. This is not ideal practice, but will do for the moment.
It certainly explains why the 3cm system had become progressively more deaf until I took it out of service two years ago.
I think I will replace the IF RG58 with LMR400 as the IF signal has to be routed on to the indoor shack. Even more loss!
Progress at last and maybe QRV on 3cm from mid week!