Hi we have a 7 month old Staffie, who was house-trained within around 3 weeks. 

These last few weeks he has started weeing in the house - we have no idea why.  Its really frustrating because 99.9% of the time the back door is left open for him to come and go as he pleases.

He has never poo'ed in the house since he has been house-trained and we really cannot understand why he has started doing this.  He can go a week or so and then just wee.  The majority of the time he will go outside, but sometimes he needs prompting to have a wee (he will wee on command) and then when he does its like he has stored it up for several hours! 

Can anyone offer any advice please?

Many thanks

He's reaching adolescence, and his hormones are driving even more than they used to, and this is causing him to feel a need to 'mark' his territory.  Go back to basics, making sure he is rewarded for doing his stuff outside, and that he gets NO attention for inside efforts, unless you happen to catch him in the act, then clap your hands to distract him, and hussle him outside......Only ever clean up using hot bio washing powder solution and follow up with a spray of white spirit.  If you use a disinfectant which turns white in water it contains ammonia, which is chemically very similar to urine and this confuses the dog, causing him to feel the need to 'mark' again in the same area.

HTH