evenin' all

i wonder if any of you kind audi fellows might shed some light on an issue im having
its that dreaded time of year where the MOT is due.
to start with I serviced the car before i took it for the test, it was sitting in the garden for a good 3 months unused.
i changed the oil and filter
changed the cabin and air filter, drained the fuel tank and changed the fuel filter, topped up with fresh fuel
my exhaust was leaking so i took that off (lovely job! angle grinder made quick work of that) new catalytic converter ... whole exhaust new

so by all accounts, emissions should be nice and low?

wrong

throughout the year i have noticed its been burning a bit, ive had to top it up a few times but emisssions were 8.5 %
my mechanic has flushed a load of additive and cleaner through and give it a good drive and its now at 5%

i can only guess it to be either piston rings, or valve seals, im certain it isnt head gasket

it isnt smoking so its not excesive but obviously needs sorting
just the usual no white smoke, no blue smoke. nothing in the coolant etc, ive checked as thoroughly as i know how to

anybody shed any light on possible cause
and if it is piston rings... how hard is the job on a DIY basis

its an audi a4 b5 1.8 se 1997