Many thanks for the info. I started with a low-power iron, lead-free solder and solid flux and had problems with dirty tips immediately. Following much good advice, I've upgraded to a good temp controlled iron, 60/40 solder and liquid flux.
I think my initial problem was that the iron simply wasn't getting hot enough. That led to holding the iron for too long against each surface and the flux discolouring and, I think, leading to the tips quickly becoming dirty. I haven't had any problems with my new kit but will bear your comment in mind if I do.