Need to refit my shock on my prophet quoted torque is 5 nm got a wrench but it only has 1/2 drive cannot convert it to take 5mm allen head.
Question 1 is how crucial is the torque?
and if not how tight roughly is 5nm , force needed ?

tight and then a little bit more.

same as every other bolt/nut on my bike.

Convert to foot pounds then use a spring balance at 1 foot from the bend in the key (slip a ring spanner or something tubular over it) – can prorata the force to the length of what you have available.

the length of the legs on an allen key is designed so that they apply roughly the right amount of torque. Assuming you have some mechanical sympathy.

One valuable thing you can do is go down a scrappy and get an old engine casing, one with loads of different sized threads in it, and tighten a few up all the way, till they strip. It's a cheapish way of finding out what it feels like.

Allen key tight.

Anything under 5 is with the small end of an allen key.

Someone once told me that the reason bigger spanners are longer is because they're calculated to be the correct torque for most nut/bolt combinations. And in general 4 finger tight with any spanner is probably as accurate as you'll ever need.

Sprocker – with the shock mount – very crucial. I had a second swing arm as a result of not torquing it up tight. The required tightness, as jambo said, is tight plus a bit