Thanks for the reply mlind. The doors are pine, and are pretty close to the colour I'd want (so I don't want to stain them).
I've got some clear polyurethane varnish that I've used on a pine cabinet before (with no sealer) and I was pretty happy with the results (I used a coat thinned with white spirit, let it dry, gently sanded it, then applied an unthinned coat).
I've used a shellac sanding sealer on some small hardwood boxes I've made (with wax to finish) but I've never used a sealer before applying varnish.
This evening I put a coat of the varnish on an offcut from one of the doors, and a coat of shellac sealer on another. I'll lightly sand both tomorrow and apply a varnish coat on each... then work out what to do from there.
I can't see why there should be a problem (I've never had any issues with the pine cabinet I made, and varnish coated). I'm still just trying to work out why the instructions for the doors advise against varnish
.