I have a WC pan from Ceramica EOS in Italy sold under the 'compact'
label by Bathstore.com.

I am installing it slightly unconventionally with a cistern (not the
matching model) in the attic above the bathroom and I therefore need
to connect the 1.25 inch flush water downpipe to the water inlet of
the pan, which is just a smooth 50mm diameter female fitting in the
ceramic. That socket is about 30mm deep.

I am guessing that I need an elastomeric grommet-style-thing that fits
the end of the pipe and mates snugly with the pan inlet.

Can anyone suggest where I can get and what kind of fitting I should
be using to connect the 1.25" PVC-U flush water pipe to mate with this
ceramic socket fitting?