Are there any guidelines as to what a council could realistically view as a S164 nuisance regarding Barbed wire alongside PROW - ie width for FP or BW?
No guidlelines I'm afraid, it lies in the judgement of the investigating officer as to if it represents a danger / nuisance to path users. If so, we'd ask the landowner to make the fence safe. A majority of the time its when a new fence goes up parallel to a (usually narrow) PRoW and the barbed wire is on the same side of the fence, we'd ask that the barbed wire gets put on the other side of the fence posts. I only serve notice as a last resort, IIRC I've never issued a S164 though.
For the status, most RoW departments would be a bit more cautious on a BW / Byway (see below for widths) due to potential injury to horses, rather than ripped clothing, for example.
Secondly, assuming that A ROW hasn't got a width recorded in the statement, are there any pointers as to an acceptable width between fences?
If the width of a PRoW isn't recorded in the statement (not many are TBH) or other documents, its width is a "matter of fact to be determined on each occasion" or indeed the width that the public have "customarily enjoyed" e.g. if people have always used the full width of a 6m wide path enclosed by walls, then the path is 6m wide, so you can't put up a boundary in the middle of it even though 3m would be enough.
Again there are no guidelines, but its generally accepted 2m for a footpath, 3m for a BW and 4m for a byway; this gives a reasonable width for two users to pass (depending on the status). So if you have an open field with a FP going down the side of it, if you want to fence off the FP, you give a 2m gap between that and the next boundary and put the barbed wire on the other side of the FP.
If you're going to put up a fence like this, I'd be worth contacting your PRoW department in advance, get them to write a letter / email to confirm they are satisfied with the width.
As cropping was mentioned above, it is (for me) an interesting dilemma. Even if acting quickly by going out there, then serving notice within 7-10 days, this gives the farmer 14 days to clear, if he doesn't, I serve notice again saying "ok clear it within 14 days or we'll do it and bill you", 95% of the time, by the time cropping problems are reported to me the chances are it'll be harvested by the time I can legally get the path opened.
Saying that, we keep lists of 'problem' paths and write to the landowners in the spring reminding them of their obligations, but there are some that know that they can block PRoWs with crops and effectively get away with it. If we had more staff, perhaps we could be proactive about it.
OSR is awful stuff, especially when it droops across the path getting you tangled up. In maize fields I always think of those horror films, one minute you're walking along then the next you hear movement or a snarl and then you're being dragged along the floor to be eaten / chainsawed into pieces / whatever