This wall is a great habitat, the limestone mortar is a soft material and is often dug out by some types of bee as you can see by the holes in the photo. Red and blue mason bees would nest here along with other bees and wasps.
I saw Blue mason bees , a queen hornet looking for a nesting site , the Pompilidae type of wasp looking and catching its prey and a Sapyga-quinquepunctata wasp which uses Mason bees as their host.
This is a partial ID ,without side shots or a sample this can not be confirmed but is most likely.
One of the smaller Ectemnius species, resembling a small E. continuus, but much less frequent.
Widely recorded in southern England as far north as Northamptonshire and west to Dorset and Somerset. There are several records for the south coast of Wales, but the wasp is unknown from the south-west of England.
Status (in Britain only)
Generally scarce and infrequent. This species is probably not threatened and appears to have increased slightly in frequency during recent decades.
Recorded from a variety of habitats including coastal and urban areas. Perhaps most frequent in rough grassland, tall herbs and open scrub.
Univoltine; May to September, with most records between June and August.
Prey consists of flies including syrphids, muscids, lauxaniids, tephritids and acrocerids (Lomholdt, 1975-76).
This is the only member of the genus which regularly nests in hollow plant stems as opposed to dead wood. Records include the stems of thistles, bramble and common reed. Up to 17 cells are placed successively in the stem, using material such as plant fragments to create partitions. E. rubicola has been reported sharing stems with other wasps such as Trypoxylon species and Ancistrocerus trifasciatus (Lomholdt, 1975-76).
Umbellifers such as wild carrot, hogweed, wild parsnip and hedge-parsley…. BWARS