Under and attached to a piece of broken flower pot.
The sac is about 2mm across.
No interactions present.
Thanks Alan. I've just looked at the other shots I took of the underside of the pot, and saw a spider that seems to fit the bill.
I've attached the pic above. Could it be the culprit?
Yes, that's an Ero species. I don't know which because I can't tell how many humps it has on its abdomen. I wonder if I can see 4 so it might be aphana.
Lat/Lng: 51.3, -0.8
OS grid ref: SU9057