I have an 18 month patterdale we have had her for 6 months now and was amazed from the start how well housetrained she was.  She can go at least 12 hours over night and lasts for the 6 hours a day that she is on her own when I am at work.  The only problem is this weekend she has wee'd twice in the house, and I don't know why because she has had somebody with her all the time and she is well walked.  Yesterday she wee'd about 3 hours after a walk.  Do you think it could be for attention, if so how can I stop this.  She is also on heat at the moment, could this be making her want to wee more?  I just don't know what to do with her and how best to tell her that it's not acceptable. Any advice would be appreciated.

It may well be because she is on heat. But it could be an infection so a trip to the vet is called for IMO. Take note of if she is drinking more than usual and if possible take a sample with you. Is she depressed and quieter than usual, off her food or more picky note these things and tell the vet

I do find if my girls ever have an accident, it is when they are in season.  It does seem they are less able to hold on when in season.  there is also the issue of marking, which is likely to occur at this time.

As said above always look for signs of uterine infection (Pyometra) when any bitch is in season or for the two months after.

Did not actually mention Pyometra but it was at the back of my mind, if could also be a urinary track infection.

My girls wee a lot more when in season, also when there season is due. As a female this is a very common thing in all off us.
If she has not been by a male the only way she would pick up an infection if she wee's a little low to the grass. Some bitches crouch, some actually touch the dirt or grass.
one of my girls once caught an infection like this, but! they wee constantly.
Don't forget the womb is larger and will press on the bladder.