In all honesty, I wouldn't know what to suggest as I have no idea what your style is really like.
I do landscape too, and I use the range from 24mm all the way to 300mm; but I am not particularly fond of wide-angle (had the Sigma 10-20mm and sold that, had the Sigma 20 f/1.8 and sold that). That said, I am now thinking of getting the
Voigtlander 20 f/3.5 Color Skopar and a
B+W 52mm Slim Kasemann to go with it.
For landscape I would always suggest a polariser filter, Grand ND filters are good too; but it's all about your individual style.
You really don't have to spend that much money, since you've got the range from 18mm all the way to 200mm covered. Remember, you are on a cropped sensor, so that's some range you've got covered.
I'd suggest look into the filters, they will open up a whole new creative opportunity. Of course, you don't have to spend the large sums required by B+W filters, Marumi make good Circular Polarisers, and Hi-Tech make good ND Grad filters. Shouldn't cost you more than £100 ~ £150 for the whole lot.
A good tripod is another thing I'd suggest, with a good ball head; this is ideal for long exposures .. and for landscapes too. That's another £100 ~ £150.
A remote trigger, for your D40, that would be another £25 I guess.
Please keep in mind that the D40 is a fantastic bit of camera. I have one, and still use it (it's in service right now) .. I loved it so much that I bought a few as gifts for family members. It's an excellent bit of kit .. do not underestimate its' ability.
. Your picture here is an example where such a filter would help with the over-exposed sky.
