Custom fields label gets outputted

But it comes in the same <span class="custom1"> for the label and the value

You are on which JEM, Joomla...(JEM responsive?)
Your last info from 2015 is

Joomla! 3.4.4
PHP 5.4.45
MySql 5.1.73
JEM 2.1 (I think, that's what the help part of JEM says)

In my version I don't have

<span class="custom1">

In JEM-Responsive there you find a consruct like this: Where you can hide the dt via css: dt.jem-custom1 { display: none; }

And as I have seen, its similar in the default JEM-View

I think we must be talking about different things.

Our page is here: www.joomlalondon.co.uk/attend-a-jugl-meeting/event/90-joomla-london-user-group-jugl

We have the latest Joomla! and JEM. At the bottom right you see venue and there you can see custom fields information.

But it's in a <span> not a <dt>

yes, this is true, we are talking about different things.
After looking into your site, I am sure, this is not an original JEM or JEM Responsive version.
I hope you understand, we can not make service to all special versions of JEM.

Sure I can understand that. I assume this is not the usual behaviour though?