Following up on Chris' find I soon located the webs - not easy to miss. With just one or two larvae visible I took one of the nests home to determine the species.
On closer examination of the nest it soon became apparent there was a nucleus of many pupal cocoons. Opening one up revealed a healthy looking moth pupa - all bodes well.
Two days on and two moths had emerged - on day three their numbers had swelled to nearly 30 individuals, they were continuously emerging before my eyes. Time to leave them outside in the garden!
The finished article - as the tree was re-identified as a Bird Cherry it's safe to assume the moths are indeed Bird-cherry Ermines. Both the size and five rows of dots also indicate this species.