He went back yesterday and sent this update.
Managed to get to Llanon again today, Thursday 8th, about 15.30, and saw two adult birds flying about, hawking for insects. They occasionally approached the nest as if they were trying to entice young birds out. No birds visible at the rim of the nest, however, as often happens if the young are sufficiently well feathered. One adult finally entered the nest after about twenty minutes and appeared to feed young birds (a very brief glimpse of one small head).
A late date certainly.
The nest is on house immediately past the very small triangular field with young bullocks on the right (before you get to the vintage bus turn). I mention this in case others passing can also keep an eye open.