That's a good point that I'd fogotten about so that risk needs to be added
to the vented tank side of the equation. After all, if you've no loft tank
then that can't happen. I wonder how how many deaths have resulted from
this? Not a lot I'll wager but then probably not too many people have been
killed in the UK by unvented cylinders either.

I seem to remember the recent infamous one (baby died on first day in
new property IIRC) didn't have the cut out switch in the immersion
so one failure and the plastic header tank gave way.