Just thought I would post my findings on this subject in case anyone has the same problem.
Turns out the self proclaimed "Pigeon Expert" at the chemist where I bought the new nipple was totally wrong. The Y cut nipple isn't fast enough if it is going flat and restircting flow and my daughter actually needs the Oral Development Nipple. But of course, no one has it in stock and it has to be ordered ':roll:'. Time to try the Avent nipple again I think!