Changed my mind about waiting until the morning and set out just after midnight, as one can get away with when looking for urban caches. FTF! Found it without even pulling out my GPS, but did have to look inconspicuous for the passing muggles (although they looked too drunk to be paying attention anyway). Took the compass (thanks!), left a candle. TFTC!
I’ll go hunt for this tomorrow, probably (I’m very familiar with the area in question: it’s right next to where I work), but based on the map this cache is by the River Rheidol, not the Ystwyth, which (confusingly) is actually a little further South. Just thought you ought to know!