Guys,
The horsey theme has to continue. Spurred on by Ezzer, waltergmw, orainsear and others, I realised that they had saddled me with afternoon of bimbling through the depths of the countryside trying to find electric fences - which I duly did.
So, taking the bit between my teeth, I set of on Shank's pony towards the next village. The bottom line is I found two operating electric fences that might be an issue. Neither can be seen from the road, which is why I hadn't found them previously. The layout of one of these can be found here:
You might just be able to see that the last section of tape on the right hand run is lying on the ground - the unducted BT aluminium cable is somewhere this side of the hedgeline you can see towards the back of the photo.
The second fence is on a spur off the main 3 phase supply and the telephone line and again has sections running parallel to both the power and the telephone lines.
The first fence (horse fence) appears to have a section lying on the ground and presumably discharging to earth at this point. Both have vegetation touching them, so presumably in wet weather...
The energizer units (battery powered) are from different manufacturers and have different discharge periods. This suggests that every so often the discharges would be in phase and so if any signal from these spark generators was coupling into the power lines, or the BT cable (by RF or by earth leakage), then every so often, there might be a double whammy that could conceivably cause routers to lose sync.
I'd love to hear any thoughts before I go off to tackle the fence owners - who, thankfully, also suffer from the effects of the REIN.