Thats the oil cooler.
Spin the filter off, you will see a bolt on the long thread that holds the filter on. If you undo this the cooler should drop off iirc. It has a rubber seal around the top. I dunno if you can get the rubber seal seperate or if you need a new cooler, but its only 20 quid ish from gsf i think.
Oh, you will also have to change your coolant when you take the cooler off, as its got two water hoses running into it.
hth