Neil,
If you operate into the UK, you have earned a portion of your salary in the UK because you have flown in uk airspace and landed at a UK airfield. They will also include your de-brief time (usually 30 mins) to your UK working. At present, they add this up, compare it with the hours you do outside of the UK and come up with a percentage of your income that is UK earned. The problem lies in the new rules that may come in, i.e. you spend more than 90 nights, you BECOME resident, you then pay FULL tax!!!! Read the proposals and email your objection. They are not about being fair. Gordon Brown wants cash. End of story.