Damien, the "green moss" is called lichen. It's basically harmless and in fact many people consider it adds character or "patina" to an older house. Don't confuse it with the cancer that occurs in coast areas where salt crystals grow inside older tiles and slowly destroy them. Yours are fine for a good while yet.
Leaves get driven under the tiles by wind. That's what wind does.Given where you're located it doesn't hurt to vacuum out the eaves every couple of years, and also check the pointing for damage.
Unless you have a real need to change the roof, don't - and don't fall for a sales pitch on recolouring (WOFTAM). Firstly, fixing requirements have increased such that you need to fix more ridge and hip tiles than some years back (similarly, certain amounts of the tiles proper need fixing to the battens) and secondly it usually generates issues of re-sarking and battening.
If you want to spend some money well, look after the gutters and downpipes, and get some whirlybird type vents installed. Aside from pest inspections and the bits mentioned earlier, that's about all you need to worry about for your roof.
Cheers, Adam.