Thanks for the comments everyone. In answer to 'doog' I would just say this:-
Just how I approach writing anything is, to be honest, something of a mystery to me. If we're talking about song lyrics (I write songs as well as instrumental pieces) then it's usually the case that an idea comes to me based on life's troubles, doubts and confusions. I then set to work to try and come up with words that sort of encapsulate my thoughts and feelings as best I can in the most economical way I can manage, and it ALWAYS TAKES A LONG TIME - many many hours of searching for and then refining lyrics that give me the feeling that I've expressed my angst reasonably clearly.
When it comes to instrumental pieces it's hard to say how and where I start. Normally I decide on a key, then start to doodle around with the main 4 chords (I, IV, V and VI) while humming any old tune that comes into my head. Thinking up a tune is as easy as breathing as far as I am concerned. What is NEVER EASY is coming up with something that sounds (to me at least) truly original. It's a case of experiment and keep refining anything that sounds like it has some potential.
That's just the start. Many hours now go by as I struggle to develop the basic idea and shape it into something that fulfills the following criteria:
1. Is it guitaristic, ie: does it fall fairly naturally under the fingers?
2. Is there sufficient interest in everything that's happening other than the actual melody, and does it complement the melody well?
3. Is there enough contrast, and in particular is there at least some counterpoint and chromatic harmony.
Above all I like my pieces to be playable and not too long!
Hope this helps.