Danfoss 103n0011 PTC is a starting relay - it has everything you need to wire a fridge compressor: it acts both as a starting relay and motor protection circuit so you can have both in a nice compact package

I got one for about 4$ - I know this part is often overpriced but it really isn't worth 20 pounds or so - if someone asks for that much it's a rip off
I've seen them for 10$ on ebay
The relay is attached directly on the three pin connector on the compressor case.
There are two types of connectors that you can find on fridge compressors:
A) danofoss connector (two prongs at the top one at the bottom - shown in one of the pics)
B) so called electrolux connector (same as danfoss connector but rotated by 180 deg)
It will work on both types - just make sure it is mounted vertically not diagonally or something
Now how to wire one ??
Here is a pic of the PTC relay seen from the top. There are 3 connectors there labled L (for line), N (neutral) and C
CLICK TO SEE THE PICTURE - for some reason it doesn't display properly
To wire one you have to connect line/hot/phase wire from the cable to L one the relay. Then attach neutral wire to N on the relay... Sounds fairly easy huh?
In order to switch the compressor ON you have to run a wire from L to C - that's where a thermostat is normally mounted - and that's where you should put an ON/OFF switch
the compressor won't work unless there is a direct connection between L (hot/line) and C on the PTC relay
EDIT
Oh and here is a video on testing PTC relays to see if they are in wokring condition
[youtube][/youtube]