Just a crazy little plot-hole filler of a thought but... When Rob gave Helen those presents for J + H - a soft toy for H I think (oh so thoughtful and concerned that H shouldn't miss out, 'cos he loves him really) - I thought 'hmm, has he implanted a listening device in there? No no, you're confusing TA with Sherlock'.
But, either Rob had set up permanent / regular camp in 'the lane' (how did no-one spot the parked car, day after day?) and on Bridge Farm itself at any opportune moment (don't they have a dog? Maybe not but a lot of people move around the farm very actively), or, he had a very good idea both of when everyone would be out and, when Jack would be momentarily unattended.
I guess we're meant to think he primed Henry with the 'leave the box in the house' routine. But, unless he had been parked in the lane for a week, just waiting for everyone else to go out (no way, too much chance of being spotted), how did he know this was the day Pat would be visiting Kirsty? Johnny safely in college? Tony and Tom doing whatever they were doing? And how would Henry know when to perform his trick?
He did track Helen's phone after all, so a bit of techy stalking is not beyond him.