I've been able to copy the example webform element as detailed at Create webform element
Although I've not tested submission yet I think it's all looking good.
What would be great is if I could clone any of the standard webform elements. I've made a copy of src/WeformElement/Email.php - and it appears in the list of webform elements available to be inserted into the webform - but when viewing the actual webform it is not rendered.
It may be that I have to use the example webform element to create a new copy - and then change stuff in there to render an email field instead - but as mentioned I'd appreciate any pointers towards being able to clone any of the elements. (The reason we need to clone them is due to styling - one text field might be used for a name - and another for a completely different purpose which will be styled differently - and as the cloned element has a new id then we can use the module themable_forms to provide more template suggestions).