Can I make a built-under oven into a built-in oven?
What's the difference between a built-under and built-in oven?
I would say none, you might need to put some support cross-pieces in the
unit at a suitable height.
that would mean there was a vertical wall of vinyl-wrapped MDF right
next to the hob. Would that need a splashback or something similar to
protect it from the heat?
I believe you need a 50mm gap even for non-combustible materials
Unless you still have the installation instructions for the hob and it
says different ...