On site www.ecolo-g.co.uk Project Galleries, with sigPro installed, none of the popup/lightbox options works, images open in a new window.

I understand from other posts that this is likely to be a Javascript conflict issue, but having been through the few mods, plugins and components switching off in turn without any result I am at a loss as to what to try next. The template index file is writable and cache is writable, while cache in Joomla is disabled (tried both ways).

Any help will be appreciated, thank you.

Try to select pop up engine -via SIG PRO parameters- that doesn't conflict with your template.
In addition I would suggest you to upgrade to SIG PRO 2.5.6.