Just had mine terracleaned as it was smoking more than the old lady in my local pub. I thought it was just over fuelling since I had the remap done. The other day it started to misfire and I thought that could be the injectors knackered. The terraclean was £60 which I thought would be cheaper than new injectors. So far it only smoked for the first mile from the garage and nothing since, even when I thrash it a bit. I've only done about 40-50 miles but even when I try to get it to misfire it doesn't. All in all I highly recommend it so far, car is loads quicker, smoother and quieter too.
if that's the case you may need yours doing every year as it sounds like you do allot of short journeys.
were you there when the terra was done as some cars can pump some smoke out of the back while on high revs.