OK, been studying the map a bit.
The lower part of Sarn Helen is still on the map as a BOAT, (although would need to check the definitive map), from Glyn Neath up to Dyffryn Cellwen.
From there northwards, it's not obviously a RoW, except for some parts that are on public roads anyway.
Presuming the lower part to still be legit and driveable, I have a plan...
assemble here Friday night, 10th October. Provided there aren't too many caravans, we can find places to park 'em. Room for tents and some spare room space in the house.
Saturday morning, drive down to the lower end of Sarn Helen, drive the BOAT alongside/through Rheola forest. Not sure of the status of the next bit - the first part is "yellow" on the map but there are various dotted white bits and RUPP lines. Assuming that bit is no longer legit (it used to be) then drop down into Glyn-neath, if it chances to be lunchtime there used to be a pub there. Head up through Ystradfellte and over the Beacons and down to Heol Senni and Defynnog and Sennybridge.
If there's time, could go and look at Usk Reservoir. There's a bit of Roman road which I have driven but it's round dots on the map, so no idea if it's legit or not. Poeple do drive it, however, not that that is any guarantee - it runs roughly parallel to the A40 and passes the roman camp at the top of the hill.
In Llandovery there's the West End cafe for a cuppa, or depending on timing, head straight back here for a barbie and a few beers, weather permitting and/or head over to the local boozer which has rather passable local ale.
Sunday, convoy over to Internal fire to arrive about 10:30, might chuck in a bit of a scenic route. Grok engines, large and small, and at some suitable point depart in search of Sunday Lunch or reasonable substitute - Aberaeron might be a good bet for that, and if it's fine there's the harbour and seaside to look at.
well, that's about it. Who's up for it?