i'm going to run the final battle from False Hope next week and I think I'm going to need to start using maps for combat, which is something I never normally do in rpg's, but I think it's kind of essential here. I'm ok with that, but I wonder if there are any decent tools that are a) free b) easy to use and c) can create the sort of environments you get in 40k (ie not just dungeon tiles or something). Thanks.

Dunno what the areas you need maps for are; but if it's a spaceship, you could try the space hulk tiles, if it's normal places, you could just use aerial photography and overlay a grid.

Actually it's for the final battle in the Akrinus Chamber at the top of the Temple. But anything I can use in the future would help. I know there's software out there, but I don't want a load of fiddly nonsense, just something that can be used to create a decent looking map, rather than a hand drawn affair.

 you could check out D&D dungeon tiles, or possibly if you can find them the star wars tiles they did they are like 6 inch by 6 inch squares normally that you assemble as you wish to make a map.