I might be able to put a photo up later when i get home.

There wont be room to extend the upper coil horizontally, as the
cylinder is in an airing cupboard that is only slightly bigger than the
cylinder itself. There is only about 1 inch of pipe visible between the
tank connector and the T for the vent, and then about 1 inch of space
between the T and the wall of the cupboard!! Needless to say, its a
pain to get in there - took me a whole day to fit a new cylinder a
while back which normally would have taken an hour at the most!!

Is there a reason why the vent has to connect at the same level as the
entry to the cylinder?

Not sure about the overheat cutout stat. I was under the impression
that 99% of the old gravity heating boilers did have this (its an old
baxi bermuda 401 back boiler)

It seems that C-plan conversions are your niche Set Square. Looking
through previous posts on the subject, your name pops up more than
most.

Cheers

Mark