Car: 2008 1.8 GT Type S
Join Date: 14th May 2010
Thanked 306 Times in 271 Posts
Hopefully just a bit of carbon deposit coming out. Other cause could be valve stem seals.
Biggest issue now, is any visible smoke is an MOT fail with a driving prohibition and even if not showing, the emissions tests are much tougher. That said, don't panic, this sounds like a one off and was probably just a build up.
If you really want to clean it out, try some BG44 or similar.