Yes, getting the tank out is a bit of a hassle as there is little clearance. I suggest you DON'T try removing the drain plug. You do need to remove the bracket that hold the boot stay, and the filler grommet. Standing in the boot while you wiggle stuff around can help. It won't come out however you try, then suddenly, it's out.
See: http://forum.tssc.org.uk/index.php?/topic/240-fuel-tank-capacity-herald-1360/
and: http://triumphherald1360.blogspot.co.uk/2012/11/fuel-tank.html
Rinsing it out while you can will prevent sediment getting ionto your fuel lines but you should fit an in-line filter anyway. Normally in the boot, rather than before the pump, although either will do.
C.