Sourdough

Has anyone had success with making sourdough bread?
I make a lot of bread so i know what I'm doing & having received Paul Hollywood's bread book for Christmas thought I'd try sourdough.
I made the starter for the first time & it all seemed to be working OK until the last day when it should have been ready to use when it looked all flat & lifeless. No bubbles & very 'wet'.
I tried again this week & it was fine- did all the things it was supposed to do- so I made the dough today & it was nice & springy & kneaded well. I left it for the first 5 hour prove but when I turned it out onto the floured board it poured out of the bowl rather than fell. I have mixed in enough flour to get it to a dough consistency again & I'm leaving it for the 13 hour prove but I'm not at all convinced it is going to work.
Any tips or idea what I'm doing wrong?
Is the temperature you leave the starter in crucial? I left it in the kitchen last time but as he advises it should be 15-25C I left the second lot in the airing cupboard. I'm sure my kitchen gets colder than that at night & never gets to 25C anyway!I did the 5 hour prove in a very slow ( lowest setting) oven with the door slightly open.
This is going to work out the most expensive loaf ever!