I saw this on Jamie at Home the other day and I thought, what with it being BBQ season, I'd give them a go. Unfortunately, as soon as I bought the lamb, the weather changed and I ended up cooking them in the oven. They'd definitely be better on a BBQ, but I wasn't going out in the rain! I found them quite difficult to handle without them breaking up when raw. They were easier after sitting in the fridge for a while, but you still have to be super careful. If you don't support the meat (ie if you try to pick them up with the skewer) they tend to fall right off. Try rolling them onto the BBQ - once they're cooked, they're fine! I couldn't find sumac, so I used the zest of one lemon instead, which was a good substitution.