Anti-pollution Fault Peugeot 807 reg no 53 plate

I have a 53 reg plate Peugeot 807 that has started displaying
"Anti-pollution Fault" from time to time over the past year. I do not want
to go to a main dealer but prefer to use my regular trusted independent firm
instead.

The first time it happened a sensor (not sure exactly which one) associated
with exhaust emissions was changed and this worked for a few months.

The second time it happened, together with a turbo charger low pressure
diagnostic fault code, the "serum" in the emissions particulate filter
system was topped up as it was low and a vacuum diaphragm associated with
the turbo charger was found to leak and was changed (the mechanic used an
internet forum to find the leak. A computer controlled regeneration cycle
was performed. This worked for about 1 month but the fault is now back on
the dashboard display with the engine management light on and the running in
"limp mode" with reduced power.

I am not expert with the fault codes or the exact bits that were changed but
remember being told about it.

The diagnostic faults read from the engine management on board computer are:
P1435 Additive system and P0235 Turbo pressure sensor.

I phoned another independent Peugeot specialist but he could not take this
on and recommended I use the main dealer as it is a known fault on this car.
He guessed I may need a new particulate filter. I prefer to let my
independent man have another go and he will consult with someone at a main
dealer for me as he has always been good to me over the years. He may
attempt to clean the particulate filter.

Has anyone got any ideas as the car seems fairly unreliable.

Thanks

Graham