Thanks to everyone who has contributed!
I drilled out the rivets, sourced new ABS corner & side mounts and have pop-riveted them onto the existing solar panel.
Just waiting for a dry day to bond the whole assembly back on the roof.
I agree with you Salty regarding the poor selection / use of self-tapping csk screws plus I don't think the original fitters pre-drilled or countersunk the mounts, just forced the heads into the plastic creating the stress.
Also when removing the old mounts from the roof there was hardly any resistance from the flexible adhesive they used and so I am not willing to just rely on adhesive alone as some suppliers recommend. I Intend to use either panhead or waferhead screws, screwed into the original holes in the roof; just got to find them!!!!
Thanks again Everyone