Hi, thanks for the replies !
I think I have narrowed the problem down to the sketch - my both-feet-first troubleshooting has got me into trouble yet again ! My initial problem occurred ( I thought ) when the lighting switched off, and the resistance of the LDR dropped. What I didn't take into account is that the sensors only read and upload every 15 seconds - and during that 15 seconds I had unplugged the 'duino from my laptop and into a wall - wart for the night. It seems that whenever the 'duino loses power , the sketch goes pear shaped. I had stripped a good bit of code out to squeeze everything in , I think I have possibly taken too much out...
In order to resurrect the sensor, I need to upload the example sketch ( tester, I think ) and then re-upload my sketch. It will all work happily again until power off. I'm thinking along the lines that some of the code that was stripped out was for finding the sensors address ( duh! ) and by uploading the test sketch ( with address - finding ) it stores the address in memory , even after uploading my own code - and works until power off. Does this sound feasible ?
In response to Kduin, the sensor module ( containing the DHT-11 , LDR and Dallas are all soldered onto a tiny 1" protoboard ) - the Dallas is an ebay waterproof S/S unit and this is then connected thru an IDC box header and ribbon cable to a corresponding connector on a shield on the 'duino.
I'll try modify the code today and see how I get on this evening.