Well after several nights and the cage being moved I started to look through lots of short videos that were captured. Many of cats snooping around, some of small birds, lots of Mary's legs. And then at around 6.00 am a gaggle of annoyed looking rooks. But they didn't find a way in. Looking further and in the middle of the night we caught the culprit - a young looking badger who seems to amble through at around 1.00am every night. He clearly has a liking of seeds and moves the cage around to get at the seed bowl - of course later on the rooks also get a share.
All very lovely but our cunning planning must continue.