Some helpful info here Bruce, thanks.
I've used the Combine software before, but not recently. I struggle trying to stack images taken with my MP-e65mm, simply because the lens doesn't focus! The only way you can focus with that lens is to move the lens either closer or further from the subject, which makes stacking very difficult. If anyone has used that lens and has successfully stacked images, I'd be very pleased to hear how you managed it.
I'll try that this software and give it a go.