Bob,
You're very right, keeping contact with the line is very important as is maintaining a continuous line through the restriction (very easily broken or tangled in such conditions). Myself and DrJM had a very entertaining dive on the wrong side of a complicated restriction earlier this year because of this.
I've often been amazed that people treat Font de Truffe in France as a training dive, often many divers' first cave dive. The entrance restriction isn't easy and the second sump is quite tight too. An air sharing incident in there would take a very cool head.
Sidemount diving is fun, you've obviously never done it right
The guy who pushed the far end of the Eastwall Inlet in Hurtle Pot said it got so tight that he ended up scratching a guideline on the rock rather than using a physical one -- hard to beat for a day out!!! There are some superb sidemount caves both here, France, Mexico, Florida, etc. It is a little more committing, but then swimming into a flooded tunnel with two steel tins of air and a few lights is pretty committing at the best of times. If you ever fancy a dip in the Dales then let me know.
Cheers,
Stuart