Have they changed the sizing in the past 16 years?


Yes. The old brown Marley are not compatible with the more modern white push
fittings. I've never bothered to check but assumed it was a metrication thing,
"real" 1 1/4 and "metric" 1 1/4.

Some of the white stuff in sheds with screw couplings has enough clearance and
bigger seals to cope with both sorts. If you can't replace it all you may need
a bodge at the join. E.g. remove the Marley O ring, force in the new pipe
perhaps with a bit of sealant, and pvc tape to keep the join together seems to
work.