I did a full set of tests 2 days ago (4 days after I added my clowns). Findings as follows:
So, this looks OK except the KH is gradually dropping and the Mg is a bit low.
I added some sodium bicarbonate to boost the KH, and have ordered some magnesium chloride. I've also done a very small water change (about 2%) when adding some more zoa frags, and will do a 10% change on Monday - I'm away this weekend.
As for the livestock, the Mushrooms look great, most of the Zoas look OK but a few are closed up, not sure why but there are a couple of things that might possibly be hydroids that could be annoying them. I tried removing them but they have come back
The clowns are doing well, they are eating well and have coloured up a lot since I got them. Snails and crab seem OK. The Pavona isn't dead, but equally I haven't seen any polyps.
I'm not going to add any more fish this side of Christmas but I do plan to put in some more Nassarius snails and maybe a couple more zoa frags.
At present I have fine sand about an inch deep and quite a bit of brown stuff on it. I'm unsure on the deep/shallow sand bed question.