Hi Thanks for the response. However sorry to be a pain but your advise is based on not having the door in the first instance & creating it from the 'hole' within the mesh. whereas I'm trying to cut my Door from existing mesh poly (which originally resulted from combination of 2 separate poly's) I've repasted the link to the sketch i did:
<center>
<a href="https://i211.photobucket.com/albums/bb28/m12ben/Efijy/CreatingDoor.jpg" aria-label="User link" rel="noopener noreferrer nofollow" class="giveMeEllipsisa" target="_blank">
<img src="https://i211.photobucket.com/albums/bb28/m12ben/Efijy/th_CreatingDoor.jpg"></a>
</center>

I've combined 2 mesh polys together where each represents:
i) the Wheel Arch (more of a mud guard than wheel arch like old style cars) &
ii) the section where the door would normally be (although this is a piece which runs from the front right through the back)

The door has to straddle borh original meshes. The polys on the resulting combined mesh are not uniform. If I simply cut a hole out, I'll end up with tris. Should I do this?

Thanks
s6x