I had the same problem, lots of white/blue smoke and a misfire at half revs plus. The problem was indeed the small gauze filter under the main fuel inlet pipe, this is found on the top of the fuel pump at the the cooling fan end of the engine. You need to remove the drivers seat, (just two nuts under the drivers seat) then move the gear lever and cover to one side (no need to disconnect cables) and remove the engine cover plate, then move the power steering tank out of the way (again, no need to disconnect it but the front bolt is a real fiddle!) clean arround the banjo bolt before removing it. Hook the visible spring out with a hook made of wire and the tiny gauze filter also. Blow all the rubbish out of the filter and re-install (it's there to protect your fuel pump!)
Hey presto no more smoke, and loads of power!
Gaz, I'm very grateful for the advise.
JON WILSON, March 2012