I take it that it must be quite tight? I am setting up my first one and I am falling in between two chain links and where I join them is not ideal. There is some tension bu the chain derails at the back! Is a half link the solution?

Are you using 3/32ths chain or 1/8th chain?
And what about the Chain ring, and rear cog?

SB

All 9speed stuff until I get it setup and am happy without gears! Split down a cassette for one of the cogs.

you'll need about 10-15mm of vertical movement in the chain. Now, most set ups will have a slack point and a tight point, so spin the cranks a bit to get the measure of it. If you're using a tensioner, pushing up is nearly always better than pulling down (the more chain wrap on the cog, the better), chain line is more important (mostly) than tension.