i'm not overly familiar with these boilers as they are quite rare but my boss seems to think they are a bag of s**t !
the only problem with desludging the boiler is that if its an old system its likely that the whole system is sludged up and if you desludge the boiler it is likely to get sludged up again. you could have a powerflush but your looking at 400-600 for that.
you could have a new heat exchanger fitted but thats dependant on whether the parts are still available or not.
or an engineer could take out the heat exchanger and try to flush it, but it may be unsuccesful and the boiler may fall to bits if its an old crock.
we did work on a very old chaffoteaux earlier this year and i know we had to condemn the boiler as it was unsafe and parts were not available for it, don't know what the final outcome was because the customer got the hump and we never heard from hm again. i cant rememeber which model it was but it was the one that has a large brass water section below it almost like a water heater.