OK, I've now got enough content, 2-3 completed chapters and parts of others, with notes for lost of the remainder. Two chapters and the book proposal are being reviewed by a good friend who is a professional writer, recently for a popular computing magazine and now for a software company. That should be a great help.
Finding an agent is a reasonably daunting task and I've been published before. I expect that I will receive plenty of rejections as its not just based on how good (or not) my book is as agents will also be thinking about how the book fits to their portfolio and experience.
Although I say "I'm sure I'll receive plenty of rejections" I actually wont, as most agents state that if you've not received a reply from your proposal in "x" weeks, they're not interested.