The combat in the game is starting to frustrate me a little and I was wondering if any of you fine folk could help.   I'm only vaguely familiar with this kind of game, and with WoW terminology and such.
 
I get that a tank is a heavily armoured warrior that draws in attacks from enemies while your ranged party members attack from a distance.  I've got two warriors in my party that I guess I'm trying to use as tanks, but they're right pussies and go down so easily that my ranged guys don't have a chance to do any real damage.
 
I'm at the point in the game where you enter Redcliffe castle, and every stupid little battle with shuffling corpses is nailing me big time.  I'm having to save after every little fight and my party is seriously injured.
 
What am I doing wrong??  I can't get the custom tactics to have any discernible effect so I'm trying to micro manage by pausing frequently and keeping all my guys on 'hold position' but it's sodding impossible.  I'm constantly seeing my guys just standing there while several enemies melee the crap out of them, and they don't fight back.  They're just standing there taking hits before dieing.  Why is that?  Is that because i've got 'hold position' turned on?  If I turn hold position off they just run about like headless chickens getting reamed.
 
Any advice would be much appreciated ...

Check out Dave's combat tips in this forum, they are really helpful and make the game much easier to understand. His username is snide and the post is on the 1st page, I believe it is called combat tips.

You can't really rely on custom tactics to get by the harder parts, at least not entirely. Explore each party members abilities and you'll find new ways to solve each encounter. A very important aspect to the combat is crowd control. If you're unfamiliar with that term, it basically means what it sounds like, controlling a crowd. When there's 6 dudes beating on your tank, they wont last long. That's when you have to switch to a character that has group stuns available. With the majority of enemies incapacitated, the group can focus on taking on one or two enemies with ease when they focus their attacks. By the time the enemies snap out of their stun, they will be much easier to handle due to their diminished numbers. 
Play around with the abilities and im sure it'll click for you much like it did for me.

Thanks for the tips!

Pause, pause, pause. Issue commands to your entire team, and make sure that you're using tactics that compliment each other. Backstabs are crucial for rogues, shield bash/pummel for tanks. Make sure to control all four members as much as possible, like the dude above said, the custom tactics are mostly there for easy to mid-level fights, on the harder or longer fights it's way beneficial to make liberal use of pause and play tactics.

Tactics, tactics, tactics.  If you've got your abilities cued up right, the game will practically play itself.  A mage's tactics should look like this:

Self: Health<75%    Drain Life 
Ally:  Health<50%    Heal 
Self: Mana<10%   Use least powerful lyrium potion 
Self: Being attacked by melee attack    Mind Blast 
Enemy: nearest visible   Walking Bomb 
Enemy: any    Arcane Bolt 
 
and so on and so forth.  If you get two mages into your party, they'll do alot of the dirty work without any input from you at all.  You might not have two mages yet, and you might not have that many tactics slots either, but that's basically what you should try to set up your characters like.  Don't forget to change their overall behavior to ranged, either, because mage's staffs are ranged weapons after all.  I would recommend not using the Hold-position in any instance except for those few where you need to lead enemies away from their group one-by-one, and going back into Move Freely as soon as the enemy gets close, or they will just stand there, for the most part defenseless.  I think the concept of a 'tank' in this game is a flawed one, not every game is WoW, and in DA you're better off attempting to not take damage at all rather than simply absorbing it.  Spells like paralysis and mind blast and skills like dirty fighting and shield bash are all useful for this. 
 
You can also make most of the default tactics settings much more viable by adding a -self: health<25%  Use least powerful health poultice- at the top of their command chain.  Make sure that you prioritize keeping your characters alive in their tactics menus, every other ability is secondary comparitively.  Just don't make the mistake of programming one of your characters for -self: health>25%  Use least powerful health poultice- or you may find youself in the dark depths of the deep roads without any health potions realizing one of your warriors has been eating them like candy, like I did. 

"I think the concept of a 'tank' in this game is a flawed one, not every game is WoW, and in DA you're better off attempting to not take damage at all rather than simply absorbing it.  Spells like paralysis and mind blast and skills like dirty fighting and shield bash are all useful for this."