I contacted UK2 support and they increased the PHP timeout settings which solved the problem.
'Thank you for contacting UK2 support. I am sorry to hear that you are having trouble with your Duplicator plugin. I am happy to help. It looks like the script was timing out on our server before it could complete. We made some temporary adjustments to our server to see if we could correct this issue. Please try your script again and let us know if you have further issue or if it is now working. We await your reply.'
I've asked for the 'temporary' adjustments to be made permanent! :-)
I really love the plugin. Using it with MAMP Pro makes developing and updating sites fast and secure.
Thanks very much for your help.
Phil