Mk 1 Focus Failed Mot On Emissions

Hi everyone,

My 1999 Focus 1.6 has just failed the MOT on emissions, (it barely
scraped through last year) with Co reading after 2nd fast idle test of 0.7 and
Hydrocarbons in excess of 300. The Lambda reading was also outside specified
limits (sorry I only have the failure notice and not the emissions printout).
The car runs great and has a very smooth idle. Mileage per gallon is pretty
good at about 26-28mpg on average and much better on a long motorway run. I
have renewed the air filter, and done an oil change with the correct synthetic
5w30 oil as part of a full service just a few weeks ago. The car does not smoke
but there is a lot of soot in the exhaust tailpipe.

The guy at the garage, whom I have used for years and is trustworthy,
said he suspected something sinister such as a leaking valve, worn bores etc
etc, but when I removed the spark plugs to check for any mixture/ oil burning
issues, the plugs were clean with only light tan/grey deposits, and this is
after 12000 odd miles since they were last changed. Like I said, the idle is
smooth and there are no hints of a misfire at any speed.

The only thing I can think of is that the "dealer" I got it
from 18 months ago had to replace the exhuast after a few days 'cos it was
blowing. I now suspect he used a secondhand exhaust and possibly also a second
hand Cat. Is it possible that the cat was coked up and the high emissions
reading is from deposits within it being burnt by the hot exhaust gasses? as
the clean spark plugs don't indicate and bore/valve/mixture issues. MOT runs
out on wednesday so any ideas anyone?

Many thanks

Rob