Help please - we keep coming back to head gasket but that is non-sensible.

We recently bought 1999 model 214i 1400 K series. It had just been given a full MOT (within 7 days) where the cat had to be replaced - proof of failure and receipt supplied with MOT.

Within about a week we noticed white smoke - thought gonna be the head gasket - bought the kit and fit everything as it should be just in case it should blow. Spark plugs were dry and clean. The gasket had slight evidence of a possible blow but no mixing of oil and water/coolant evident - the head was smooth and clean. Everything else was good - replaced all with the new kit and new coolant. cam belt good. Air filter clean and good - changed the oil and the oil filter obviously. manifold gaskets changed too.

Still got exactly the same problem - exhaust billowing when started up and constant but not hugely evident when driving. Looks white but is smelly with exhaust fumes - what to do next? Used Kseal - made no difference.

Please help.