Guys, let me just say that I have well and truly learned my lesson from this. Like Baz says, the safety of the fish comes first and if there is a risk of this happening again I will reel in for the night (or go home).
But a couple of points to note.
Before I set up I took "local advice" on where was safe to fish...from two bailiffs who assured me that this section of the lake "never freezes over".
There was no ice on the water at 17:30 when it got dark.
In the morning when I got up, the same two bailiffs were as surprised as me, and if you haven't already guessed, they were in the same situation. I had to use my lead to free the lines for one of them.
These guys have been fishing the lake every weekend all winter (except for Christmas). According to them, they have never seen that part of the lake freeze over before. The same comments came from a few guys who turned up early Sunday morning.
Luckily this time there was no harm done and I will not take the same "risk" again.
I was not trying to justify what happened in any way, but trying to find out if there was a better solution once I found myself in that situation.
Anyway, I will leave it at that, except to say Baz, that I will be more than happy to buy you a pint if our paths cross.
