Ed Sirett wrote:

These are very tricky to fit. However you should have found inside the
packet a small steel slightly cranked rod about 2mm diam x about 50mm
long. This is used to wedge the chain. You could also improvise
something.

Yup - use a vice & mole grips to pull out 2" of chain, and then lock it in
place with the supplied rod (sometimes a notched steel plate is provided
instead). The resulting 'slack' allows you to fix the spring barrel into
the door, and then the plate onto the frame.

Having done this, you'll probably then need to take it off again to wind the
spring in or out, adjusting the tension. Open the door wide, put your
locking pin or shim back in, unscrew the plate and then wind the chain with
pliers as you need to.

I find them perfeclty usable for domestic doors, but they don't usually give
you a full fire-door 'slam' unless you wind 'em up tight. I don't like the
noise, so ours are adjusted to pull the door 'nearly closed' without
latching.