On Aug 15, 9:53 am, fred wrote:

Try part no 16148, 82 mm, black, strap boss, 2/3 down this page:http://www.bes.ltd.uk/products/130a.asp
Boss adaptors here, top of the page:http://www.bes.ltd.uk/products/130b.asp

But a local plumbers' merchant or drainage supplies place should do the
same. It is 3" nominal so that is what you should be asking for.

Thanks. It's exactly 3.5 inches external though - I'm not sure if
that's the same as 3 inches nominal.