my five month old cocker spaniel bitch has amazed us at how quick she seemed to get house trained so image my surprise last night when my 17year old son returned from taking her for a walk and the first thing she did was to jump onto the sofa and went for a wee!!!   We do not allow her on furniture but she often jumps up to chance it, I never smack her but give her a firm command of OFF when she is on the sofa.  The seat is saturated and its foam filled - what should i do to avoid her doing this again? - she is our first dog and I've read that 6months of age to 18months are the hardest in having a dog as they are at their naughtiest.

Any advice or tips on how to deal with this will be greatly appreciated.

To prevent this becomming a habit, I'd keep her on the lead when she gets home for a walk, and let her in the garden first.......
Secondly, wash the sofa with a solution of biological washing powder and let it dry outside (away from the strong sun) test a small area for colour fastness, and spray with white spirit or white wine vinegar - this will break down the fatty acids that hold the scent that may cause your bitch to remark the same spot.......

She's coming up to the age where her hormones will start to govern her behaviour, and many bitches start 'inappropriate marking' as they approach their first season :(  If she's prone to this, then management is the best way around it (not allowing her on the sofa, into the garden on  return from walks etc.)