Dave, sorry I can't remember what the final block manufacturer, it was an Ebay special I bought some time ago for about £20.00 brand new, I think Lewmar springs to mind. Most of the blocks I use have been Holt Allen, great blocks at low low prices. The through deck block is http://www.force4.co.uk/ProductImages/088.jpg, Harken xxx although this is a really low tensioned line, these seem to last forever without really ever failing.

Inside the boom I have a 30mm at the front against the gooseneck and a 30mm with becket at the rear creating a 3:1. Not really sure whether this is a 12:1 or 9:1 you can discount the last block at the front as it has very little effect, but it is nice and easy on the forearms.

There are a number of do's and don'ts with cascade systems, they can be a bitch to set up to get maximum travel, but worth while.

The biggest don't is to make the rear boom block at the back fixed inside the boom. The mainsail with tension on can grip the boom and turn it just a few degrees beyond the best working angles causing alsorts of binding problems. I solved this by using a tie on block mounted onto the outhaul pulley, the tie on allows the block to always be at the best angle.

There is an interesting discussion on SA re the mid mounted sheets for A classes, lots of interesting litle nuggets of info there.