Shelby3 is correct it's an engine breather / oil catch tank. Yeah, ain't they nice ali welds, it was seeing the quality or their work that led me to approach them to custom make me a system (rather than make something myself only half as pretty!) the tanks fully baffled, and has a sight glass, and tap to empty the oil out. Power steering cap breather is also plumbed into the catch tank; it takes a small flat K&N filter.
Stand by for the polished coolant header tank they are currently making me (to replace the plastic OE one)! Several of the UK guys have already put their names forward for one (...polishers)! I'll ask re prices and post some pics, if anyones interested? They are also going to do a windshield washer bottle and fuse box cover. I'm not too into all this dress up stuff, but I have to say it does look good and is perfectly fabricated!