Look at the wires on your old loom and make a note of where they go in relation to the plug (the plug can only fit one way). Then pay a visit to these people:
http://www.vehicle-wiring-products.eu
They sell the correct male and female connectors for plugs - they have a locating latch to hold them in the plug.
Buy the ones you need for your plug.
Get a small screw driver and bend the locating latch flat on the connections in your old plug and then push out the connectors from the plug.
Strip the ends of the wires of your new loom and crimp the new connectors on. Push them into the old plug (in the same relationship as your old loom) until the latch clicks into place. Job done.