Wow, thanks for the replies guys! Your points have given me a lot to think about.
Uvenlord and MikeT - You guys are correct, my opponent does use up a lot of his points in doing that - but I find over the course of the game with him or my Ork friend that it's the loss of 1 or 2 activations from my army so early on that I then start to struggle with. The assault marines all die later alright, but I usually spread out my deployment to cover as much of the board as I can, and to advance evenly on all fronts, to make it easier to nab objs in the late game. This means that I don't usually have much more than 1 or 2 other units near enough to react quick enough. It also slows down my advance, and keeps most of my remaining units stuck in my deployment zone until turn 2 when they move off again.
Which leads nicely into the next point that Dave bought up - I like the deployment there, matey. I will have to try that sometime - as my opponents ALWAYS have Air Assault units, i'm probably suffering a bit from inflexibility in my tactics - i'll see how a really 'cramped' deployment can serve me. hmm.
To those who pointed out that I should be getting 6 attacks not 4, you are correct - that was me thinking (and typing) too fast and not checking my maths, only my spelling

Although 6 attacks on 5+ would average 1 dead marine, however I also didn't add the Thawk to his numbers, so it'd still be 8-1 or thereabouts.
Lastly - this more concerning point that Jim has quite rightly raised - it seems some of you think that 3.3.2 means I could use my FF attacks on other stands not in base contact with my WE but still in base contact with other WEs in the same formation. 'You may choose to split these between close combat rolls and firefight rolls as you see fit, but close combat rolls will only hit enemy units in base contact, while firefight rolls will only hit units within 15cm that are not in base contact.'
Surely that (while admittedly ambiguous) point means that if I've got 3 knights in a unit, and they are in base contact with a unit of 4 ass marines, themselves all in base contact, I cannot use this rule? My group have always viewed it as FF attacks can only target (and be assigned to) other FFing units in the same combat, while all CC attacks can only be assigned to units that are themselves in CC.
Because this is possibly why i'm struggling in CCs. Are we interpreting this wrong?
-WoU