The same evening, I emailed our Allotment Officer to tell her what had happened. On Monday, I tried to phone her as well, but she wasn't there, so I left a forceful voicemail message for her. About an hour later, I got an email back from her saying she'd asked the contractors to put up fencing between the Family-from-hell's house and mine and my neighbours' allotments. This would be the high, green metal fencing that they've used to fence round most of the allotment, so will be plenty high enough to deter these scavenging scroungers.
I told both the Joes and Franco about it. Both the Joes called the FFH 'bastards', and Joe #2 (no. 58) told me they'd pinched his tomato stakes on two occasions, and he'd come close to "putting a fork through their neck" when he'd remonstrated with them. He said he knew it was wrong but it would have been worth it. You'd never guess to look at him - he's as mild-mannered as they come. But a lot of these Italians served in the war, and would think nothing of stabbing a man. Franco pretended to be more laissez-faire, more philosophical. Perhaps because he's got so much rubbish on his allotments he wouldn't notice if any of it went missing.