I've got a new Grohe mixer shower which I need to fit, and it comes with a couple of "S-unions". One end attaches to the shower mixer, and the other end to the pipes. However, there are no instructions on how the pipe connection is supposed to be made.
The S-union is threaded, with a 15mm diamter, so I initially thought it was a compression fitting, but the inner diameter is too small for a standard water pipe to fit into.

Does anybody have any experience with these connectors, and any idea of the best type of connector to use?

No idea what an S-union is, but I fitted a grohe shower using tap connectors and significant amounts of PFTE on the thread...

The s type fitting you describe has a 1/2 inch BSP thread on it and you would need to buy a straight or bent female iron X 15mm compression fittting.

Mog