Hi Rob, firstly nothing really to worry about, you do get a fair ammount of "blow off" oil in the plenum, just take it off, wipe out the inside of the plenum and then tighten up all the bolts again, be careful though on the tightening, dont over tighten them... You might just find the bolt is loose!
Take some pics of the inside of the plenum when you do uit, there shouldnt be loads, but a smattering!
Then just keep an eye on it. Cant tell from the pics but guessing its the middle back bolt hole?