My six month old tibetan terrier has been housetrained for a while and is fine during the day but has had a relapse over night.  We now come down in the morning to a mess in the kitchen.  She goes out at 11pm when we go to bed and usually wakes us up at 6am by barking to be let out.  I would appreciate any advice on this as it is very strange that she was clean at night but now seems to have changed. Thanks

Have you considered crating her?  Dogs will usually not toilet in a crate because it is a restricted space and their bed....

http://www.ddfl.org/behavior/crate-train.pdf