No nothing, just MDB running on asp.net. And the odd thing it works fine at www.ovl.co.uk which is similar site for same company.

I have tried that code on http://beta.vans4lease.co.uk/contact but it still does not work. Works fine on HTML page not asp.net. But it works on asp.net page here https://www.ovl.co.uk/contact

Maybe I will have to disable the MDB checkbox and have a standard checkbox. How can I do that?

I have updated the Label for ID name but still not selectable. Any ideas?

<label class="custom-control-label" for="MainContent_CheckBoxMarketingFP">I would like to receive the latest Vans4Lease offers</label>

There's one problem with your code. Let's take a look at your code:

<div class="custom-control custom-checkbox">
    <span class="custom-control-input"><input id="MainContent_CheckBoxMarketingFP" type="checkbox" name="ctl00$MainContent$CheckBoxMarketingFP"></span>
    <label class="custom-control-label" for="CheckBoxMarketingFP">I would like to receive the latest Vans4Lease offers</label>
</div>

Your input's ID is MainContent_CheckBoxMarketingFP while the label is looking for CheckBoxMarketingFP. Everything should work after you change the "for" to the ID used by your input