Can anyone remember the RTS game where you had a mech robot that you could customise with a plasma sword and shield, or plasma blasters etc. You could engage the opponents mech and if you bashed it up good enough you could make it lose weapons that you could salvage to your tech building and assimilate it to create your own uber mech?

Now you could build troops/vehicles but it all centered around your leader mech. It's not the Mech Commander series or Total Annihilation series. I think there were 3 factions - brothers according to the story if i remember correctly.

Thanks for your help!!

the only thing i can think of aside from what you mentioned is dawn of war, but i don't really remember if you could salvage weapons

Metal Fatigue?

Pretty likely.
Decent game with horrible pathfinding.

Couldn't remember the name, but yes you're description reminds me of Metal Fatigue as well (played the demo to death) I found it again on this list:

http://www.giantbomb.com/profile/lazyaza/lists/the-10-best-rts-games-of-all-time/23880/

which I post because its a pretty damn awesome list!
edit : hmmm or not : there's no reason to include DoW II over the the first DoW :P