SPO - Modern Lists with content types and targeting the CT in the edit form

Hi,

So I am trying to use new modern list layout as it formats well on mobile devices.  In the "old" days I could do everything in InfoPath and the form would be built in 10 minutes.  However I've been trying to get this working for a few hours now and I'm still not getting the result I want.

So the issue is I have a custom list that has a couple of content types.  These content types control what fields are available on the form when a user creates a new item.  I can target the content type easily enough in the NewForm URL by using ContentType= within the URL structure.  When it comes to editing the item I can't do the same level of targeting, it looks like the ContentType parameter is ignored when EditForm is used.  On the edit form I am presented with a drop down that allows  the user to switch between the content types, this is ok but I'd rather users didn't have to worry about selecting the right CT and the edit form just showed the CT that I desire. 

I did think that maybe changing the default CT would mean that CT would be the one shown on the edit form, however this doesn't seem to be the case and the same CT (the first that was created) is always shown.  The other thing I tried was to make a new edit form in SPD and associate that with a CT but that  drops me back to the old style forms.

Any thoughts gratefully received.

Rob