in 6years i´ve not came across a S1 car with an orginal chassis, where the rear corssmember was not rotten.
even those cars looked very nice, and most of them had a fresh british mot!!!
the most formative experience was my first eclat: i bought the car in UK from the first owner. the car had 30000 (original!!!!!!) miles and the chassis was replaced 2 years b4 my purchase. the car was 28y old. pictures where taken when the body was of for replacing the chassis. the rear crossmember was missing on around 70% (view from top), the coil-seats rotten through.
that opened my eyes...and every thorough inspection afterwards, of an original S1 chassis resulted in, more or less the same result!!!
NEVER,EVER buy a car with a non-galvanized chassis!!!