All the slate roofs I have done have used the Unirac system (available from Dulas) which is simple to install, won't leak and doesn't involve seals (or worse still, silicone sealant!!). I found it really easy to install, as a non-roofer and the results were robust and very pleasing. I can post photo if you want to see it. It's much faster than forming little lead pieces as well.