Hi Malcom,
I think the answer to your problem is indeed in the link Serge provided, but for the sake of clarity, here is a summary :
Identifying in which template is located a particular element of your website is a common problem, and not so trivial.
The templates used in your website can be the default ones (in "squelettes-dist"), or your own (generally in “squelettes”), or the ones from a plugin (in “plugins/xxx”), depending on the configuration of your website.
Moreover, a template can include any other templates, so in the end the templates in use on your website can come from a lot of different locations.
Fortunately, as mentioned in Serge’s link, SPIP provides a very handy tool for this purpose.
You just have to append ?var_mode=inclure
(or &var_mode=inclure
) a the end of the url of the current page, and you will see yellow boxes detailling all the templates in use and their location.
For this tool to work, you need to be logged in and an administrator.
So, concretely, go to www.chelmsford-tuc.org.uk/?var_mode=inclure
and that’s it.