Sorry for the late replies! I had no idea people would be so interested in this piece. Thanks very much Sonata and Nightscape, I really appreciate your comments and it's so great to know people have enjoyed my piece so much!
It was quite a long time ago now (April 07), so I can't exactly remember how I did it, but as far as the viola line is concerned, I think I just started out with the C#-D-F#-C# motive and played around with developing it, mostly at the piano to begin with, which got me up to about bar 14. Then once I got to the cadenza, I came up with a sort of chart, all based on the opening shapes and intervals, and I guess I came up with quite arch-shape phrases without really thinking about it.
Then in the tango I just recycled most of those ideas, but put them into a different order, and concentrated very hard on getting the kind of rhythms I needed to make it exciting, stealing a lot them from Piazzolla, but slicing them up and rearranging them as I went along, as well making up some of my own.
I guess the main thing when I wrote it was that I was very aware of what I wanted it to 'feel' like, before writing much down. I wanted the viola part to sound very lyrical and free in the elegy, and for the lines to sound very continuous and expressive. Later when writing the tango I knew I had to use the same material, but I wanted it to still feel quite dark, but exciting and perhaps a little bit sentimental in a kind of nostalgic, over-the-top sort of way. I remember I actually found it sounded quite funny to put those sorts of harmonies with the rhythms I was using - like in bar 18 for example.
In the end I was very happy with the results though...
And I'm so glad you've all thought that too. My girlfriend (who used to listen to lots of tango, she'd learnt to dance in Argentina) who it was sort of dedicated to (I only told her when I knew she liked it) thinks of it as a kind of parody of the intense pride and emotion that you find in the style. But this is all b*llsh*t, I think that my own analysis is only as valid as the next person's!
So pleased it's caught people's ears like this!!