Hi Mr Marino,
Thanks for taking the time to look at the numbers, that's a lot of hours! I'm happy to have a relatively large stepover - and accept the loss of finish - if it makes the machining time doable. Jonathan - who is also looking at the file - has very kindly offered to do a small test piece based on a 0.1mm stepover to see what it looks like. I can't wait to see it!
The French company sounds promising, do let me know if you find their details. In the meantime I'll have a look at the weekend at the program Phil suggests (thank you Phil!) and will post any news...