Have changed the old LX rear mech on my Scott Reflex for a Shadow SLX. Followed the fitting instructions, checked chain length (largest chainring / largest gear on cassette taught plus 2 links)now when on smallest chainring and smallest gear on cassette there is no tension on the chain – chain just hanging slack. What am I doing wrong?

well, that's the rule of thumb, presumably you did measure that out without looping the chain through your mech ( just checking! )?
now, see that bit of metal between the bolt onto the mech hanger, and the mech proper? that's where your new mech will break…..

yup – measured without looping through the mech.

You've probably got the GS version with the shorter cage, it's less capable of taking up the slack in the chain than a longer one. Aside from changing it, you'll have two options:-leave it and don't run a small/small gear,or shorten the chain and avoid running big/big gear and it's ensuing destruction of anything that'll "give" connected to the drivetrain.Due to the awful chainline, they aren't gears you should be using anyway, so probably not going to cause too many issues.

Agree you shouldn't be running small/small anyway. Take one link out and just make sure you don't shift to big/big

thanks Coatsy/Capt'n M – def not the GS and having just spent 15 mins sat on the floor at Treadz even the new bikes with the SLX have very little tension on the small/small. I'll just lose another link.

my first shadow mech was too tight on the pivoit bolt, loosen it?

Do they have a b screw to adjust the distance top jockey wheel to cassette? Is there a massive gap? If you shift into big-big, do you have any slack to remove another link? SRAM chain and you didn't count the powerlink as one of the extra links? Otherwise you might need a longer cage by the sound of it.

Fitted a new one yesterday and tension seems fine small to small. I always set the chain length by putting on big sprockets front and back and pulling the mech forward almost as far as the spring can take, then cutting to that length to include the connector on Sram chains.
I didn't have instructions and have fitted with the main bolt to pivot bolt horizontal, which I hope is correct.

I have the same problem, a slack chain on small to small

who rides small to small, I’m sure centre to centre would give as gear of a same ratio? small to small is the worse idea ever because it wears the chain quicker.