Both of the sites I usually use for finding access roads - Google Maps and Multimap put the NGR for Mynydd Emroch in the middle of nowhere. The site is clearly visible as you travel west across the M4 from just after junction 39.

The site looks down over Port Talbot, which is pretty well covered on all sides by mountains (being as this bit of Wales is called "The valleys".

The NGR is bang on the money, but you need to do the usual looking for unmarked/unnamed/etc roads to find the access road. Usual clues gave it away: 3 phase overhead lines seemingly going nowhere, substation in
the middle of nowhere, and "the feeling".

It was a bleak day, and there was a lot of low cloud and frequent showers which brought the visibility down somewhat.

There hadn't been any recent activity at the site - very tall grass having grown across the access road, and chains at the gates heavily rusted in place.