99% sure it's the length as when I tried the PC closer to the TV with a 2m cable it was fine.
So far I've tried:
Decent quality but old (iirc HDMI 1.4) cable I already had - connected fine, dropouts every few minutes
Cheap HDMI 2.0b cable - constant flickering
Decent quality HDMI 2.0a/b cable (the one I'm currently using) - connects fine, dropouts every 30 mins or so
Powered repeater - same as above, connects fine, dropouts every 30 mins or so
Active copper cable - constant flickering and noise
Yesterday evening Amazon delivered me a "Mavislink" fibre cable (best reviews & longest warranty around the £50 point), it connects fine, no noise or static, but haven't had a chance to test it properly gaming yet.
Fingers crossed it works, otherwise the next option is spending £££ to convert my PC to mITX to go under the TV and seeing if the connection works the other way (or buying a gaming laptop
)