jonb: Thanks, seems like will need to raise budget or do in increments. Do you have experience of getting builder, plumber, fitter, tiler individually compared to a one stop bathroom shop?
Having seen what was involved, it would largely depend on how you value your time. We did it with a firm who handled everything and given how many issues and how much time they spent for the fee they charged I am VERY glad I opted to not do it myself (Project Management).
Don't do it in increments, do it 1 bathroom at a time completely. The number of ways an incremental upgrade could go wrong are too numerous to list.
I'd recommend (strongly) Increasing your budget and saving longer than doing it piecemeal or on the cheap.