Thanks for the quick response!
Having looked at the error_log on the destination server it look’s like it might be a memory issue:
[Sat Feb 14 18:04:21 2015] [error] [client 18.104.22.168] ModSecurity: Output filter: Response body too large (over limit of 524288, total not specified). [hostname “traintocampaign.london”] [uri “/wp-admin/customize.php”] [unique_id “VN@OIX8AAAEAA5ZGG4IAAAAX”]
I’ve passed the matter on to the host to deal with.
Thanks again. Awesome theme, by the way!