Perplexing gear issue

[ElliotR]

Lo there
I just bought a new bike and assembled it myself, always bought them from local shops fully set-up before, and have a slight gear problem.
Initially I had a problem with slipping down cogs when changing gear that was sorted by adjusting the cable at the shifter end. Also after watching the very helpful vids on this site I made a few more adjustments that have made my gears nice and smooth, wouldn't have had a clue without them.
The only issue I have now that I can't work out is I can't get the chain to stay on the 2 largest gear on the rear cassette. I have absolutely no trouble with selecting any other gear but this one does not want to play ball. I can see no damage on this gear and as the bike is new I doubt it's wear so any idea's from anyone?
In case having some idea of what gears my bike has helps in anyway, here's a link to it....
http://www.winstanleysbikes.co.uk/product/14400/Dawes_Discovery_401_Bike_2008
anyway, thanks in advance for any help you can give me.
cheers

[DaveM]

When you say "can't get to stay on", does it shift into those gears but then drop back to the thrid cog when you let go of the shifter? Or can you not shift into the two largest at all?
When you shift to the smallest cog in back, is there a lot of slack in the cable that goes to the rear derailleur?