I've just been given an air compressor but it states in the instructions
that it needs a 15A supply and I'm not sure what the best way of wiring this
in will be.

At present I have SWA cable running from a 20A MCB on the CU into my garage,
this terminates in a metal switch box which in turn has 2 supplies running
from it. One is 1.5mm to the lights the other is 2.5mm to a metal socket
which is mounted right next to the switch box (physically connected to it
via the gland hole)

I was hoping to run a 2.5mm cable from the socket to a new 20A switch (run
of about 3m) and then connect this switch to a round 16A IP44 socket.

Graphically it would look something like this

SWA----- Switch - Socket ------------Switch - IP44 Socket
|
Lights

The first socket currently runs a fridge freezer.

Would this set up work or am I likely to burn the house/garage down. Would I
be better to connect the new 2.5mm cable direct to the SWA terminals inside
the first switch (It's tight in there already) If necessary I could wire the
compressor directly to the second switch but liked the idea of the
versatility of being able to unplug it.

Any help much appreciated and hope my explanatipon makes sense

Jim