After much experimentation I managed to get a really active starter going. Here we go, I thought! I kneaded for 15 min. The dough worked nicely and I felt I was going places at last. I let it rest for 4 hours and it rose, my how it rose! I transferred it to a colander, covered it with cling film and left it overnight in around 27 degrees. In the morning it was up like a balloon but as soon as I removed the cling film the whole thing collapsed. Needless to say the resulting loaf was not the best, even the ducks rejected it.
Where did I go wrong? I guess I let the second rise go on too long but how to judge when to bake?
I'd be glad of advice.