Hi,
Reply to your post 26/5 the screw heads:-
You can drill the heads off which will enable you to remove the tank.
You can then either use Penetrating oil, heat, an ezi-out, or drill and re-tap. The original bolts were 1/4 WW. with a star washer underneath. I've replaced these with 1/4 WW roofing bolts which are easily sourced.
Hope this helps,
LS
