I’m going for the 20mm blocks too. I was leaning that way to start with, especially after David’s helpful comments.
Meanwhile, I also e-mailed Holt-Allen to ask what they’d advise, not really expecting a response. But this morning I got an extremely helpful answer from them, covering every point I’d raised with them and recommending the smaller blocks as being plenty strong enough and cheaper.
It’s really good to see companies giving honest advice in this way: they’ve certainly won my business!