Brilliant guide - many thanks.
Fixed it myself at weekend, only took about 15 mins. Replacement LED was £41 inc VAT from local dealer. Also used a 5 piece set of BOJO trim tools bought from everyones favourite auction site for about £10.
Here are some more photos which might give others the courage to go for it ...
To get started drilled a few holes in one side of the led light casing (just be brave and go for it)
Next, get your trim tool of choice and start cracking off the red platic casing. Might be worth wearing some eye protection here as the shards of plastic ping everywhere. No real technique just get your wedge under the plastic and brake it off.
With the plastic removed now get your trim tool in between the white plastic casing which contains the LED's and the hole in the spoiler which seats the light. I just prised at both the far left and right ends of the light and after a fair bit of wiggling about you will hear the plastic which is holding the retaining screws (see below) crack then you should be able to just lift the light out.
After removing the main LED housing as per step above you now need to get to the cable which connects it to your cars electronics. What I did here was get a pair of needle nose pliers and feed them into the hole where the cable is housed (you should just be able to see the plastic rectangular part of the cable buried inside the hole - in this picture the hole i refer to is the one in the centre of the picture with the wires dangling out). Just use the pliers to grab it and yank it out through the hole.
Also in this picture you see one of the retaining screws which I mentioned above (rusty screw in far left of picture). In your new light you will notice that there are 2 holes in the back of your brand new LED. If you were doing it the correct way (i.e. paying VW £300+ to rip off your spoiler :-P) then you would be using these. However for our purposes we don't need them so just use your pliers to twist and lift them upwards and they should come out. Disgard them once removed as they are now useless.
Bit blurry but this is where you connect your new LED up to the wire which you prised out of the spoiler above. Must admit it confused me for a bit as to how to detach the old cable but if you look at the picture you can see a very small platic notch (just above and to the right of my thumb) - this twists slightly allowing you to pull out the old connector. BEWARE : this notch doesn't feel very substancial so be very careful.
Dont have a photo of the next step but what you need to do is connect your new led to the cable from the car, feed the spare length back inside the spoiler and then refit the complete LED housing back into the spoiler. Its not immediately obvious how to best do this but if you push the LED back inside the spoiler it will eventually click into place and should hold. I think I might add a drop or two of silicone sealant just to be sure that its properly held in place - remember the LED is no longer held in place via the screws which were inside the spoiler. (will also help to prevent future leaking)
At this point you should now have a broken old LED light and a few thousand shards of red plastic.....
But finally here is what you should end up with - a nice new shiny LED and with the £300+ that VW would charge you still in your back hipper !
Once again thanks to original poster (ub7rm) for all assistance provided