Hi Ali
..try a small torx drive screwdriver or bit - I've had some good success using them in the past when mostly everything else has failed... angling it very slightly towards you to get some pressure on will help too - and before all that give it a real good drench with release spray and let it sit for a decent amount of time... also, try different allen keys of the same size... tolerences are everything at that scale !
..the only other thing i can think of is an easy out, which if you can find a small enough one should pull them no problem.. and having seen the tank off they really only dig on the pointed end very slightly into the alloy housing (hence the 8 probably) - so possibly a bit more 'not so gentle' persuasion could be in order... but that's a toss up between it coming out and deforming the tank ... hope that helps
Cheers
Dano