Hey ho!
Before going out to buy armfuls of cable I tried moving router as far from PC as possible - given length of cables.
Powered up router via extension lead from another socket and tried shielding router with tinfoil
(don't laugh!!)
Good news: turning off monitor showed no improvement - so - don't need a new one of those presumably.
Bad news: turning off/on PC had same drastic effect, even at maximum distance (admittedly only about 1.5 ish m - without ethernet cable connected.
(Am now wondering if this thread ought to be posted on hardware forum?? - no doubt will be advised - if so apologies to all.)
So. . longer master socket - router cable to be purchased. Not sure about buying ethernet cable yet, as if it turns out I need the router out in the shed to be 'protected', then I may need a 20m one rather than 'just' a 5 or 10!!
Before anyone suggests throwing out the PC - don't think I haven't thought of that!
You would have thought PCs & routers could be mates wouldn't you?