After dewaxing and flatting off with wet & dry, which type of primer should I use - Etching or normal

use a good quality etch primer and don't flatten the GRP suface too much, 400 grade is as fine as you really want to go as the primer will have a better key

ordinary primer will come off in sheets

As above, etc primer on a keyed surface worked for me.

Halfords actually sell "Plastic primer" in cans which is advertised for use on fibreglass as well as plastic. Has anyone tried this??

yeah the halfords one works fine, I primered the falcons wings in it and it showed no sighs of a bad bond. Use light dust coats at first, then build up the primer thickness and flatten it with 600 grade ready for the colour coats