Sorry about the late reply, I've been tied up :)

I did as you suggested the other day and unfortunately, it doesn't appear to fix anything.  My LAN drivers are up to date (one of the steps I'd already taken) and the cable appears pristine; it works perfectly every time when plugged into a secondary router and to my lappy. 

I've been able to all but confirm that it's my PC; I just need to unplug the cable for a few seconds, plug it back in and half of the time that works.  If it doesn't, I just do it another couple times until it does :P