I had the same problem, also on heavy clay. I put it down to vine weevil
despite not being able to see any. I figured that maybe in open ground they
could move around so were less visible. Woudl be interested in knowing how
much of a problem it is likely to be though. I thought that garden plants
were supposed to be able to withstand them.

--
Hayley
(gardening on well drained, alkaline clay in Somerset)