Can anyone tell me what the black plastic box is that sits immediately in front of the windscreen washer fill up on a 1.8tfsi?

That's the charcoal canister as far as I know, something to do with venting the fuel vapours from the tank,

I believe it's so no fuel vapour enters the atmosphere and instead is used up in the combustion process.

Ah thanks. Mice decided that it was worth chewing and chewed the cap/fitting closest to washer fill up. Not sure whether it is a replaceable item or whether I'll have to get the whole unit.

It's replaceable. I had mine done a few months ago after my MPG dropped I thinks it's around £80 fitted

How does it effect MPG out of interest?

Mine has dropped recently... But I put it down to the cold weather?

Im not really sure how it effects the MPG but thats what was at fault and upon its replacement the MPG shot up to 33mpg and not the 19mpg i was getting before.