I put one coat of undercoat on the one of gloss. In places I
could faintly see the previous colour through the new paint if you
looked closely.

Acrylic undercoat isn't anywhere near as opaque as a solvent based
alternative. The opacity is found in the top coat.

Is Glidden Trade Acrylic any better in terms of opacity.

I use Glidden Trade Acrylic and Glidden Trade Acrylic Primecoat
Undercoat/Primer. I like it, but I don't expect the undercoat to have any
obliterating power. It is basically providing a base for the real paint,
which does.

I hope you've truely rubbed down the old gloss to give a good key. Water
based paint will not stick to shiny old gloss paint.

Christian.