I see you're UK based. Take a look on the Pentax User site unless you're a member already -
Pentax Forums - Pentax User Photography Forums.
Plenty of gear for sale on there. I'd look for a 15 or 21 Ltd for your landscapes, or a DA 12-24. For something slightly quirky, a DA 10-17. Real fisheye effect at the short end, but great wa as it is zoomed towards 17.