The phono stage on my Roksan has started only playing through 1 channel.
It looks to me like relays do the input switching on your amp (you will know that's true if you hear clicking as you change inputs).
It's likely that the phono input relay is only successfully switching one channel.
Either its contacts are not connecting, or there is insufficient voltage energising its coil.
If you (or someone you know) is confident enough to avoid the live mains within, it would be worth getting to the relay pins (on the solder side) for some deliberate short- circuiting and voltage measurement.
If not and it's convenient for you to get the amp to South East Essex, leave it to me 👍