I have removed the light cluster from the car hoping to remove the lens and clean all the muck out of it, but it appears to be a sealed unit! Any help much appreciated
Yep its a sealed unit alright ! I know this as i had to take one of mine out the other day as a bulb had fallen inside it. You could try soaking it in hot soapy water i suppose to soften the crud and then flush it out.
Remove the 2 nuts you can see under the boot lid on the edge and then pull the internal cover/carpet away and you will see another nut holding on a sort of clamping arangement, remove this and then push the light fitting out, dont forget to disconnect the wiring though.