I think its fair enough your probably best to just accept the fine, and be glad that no points where involved.
The tax disc wasnt displayed, as the woman says, end of lol. Am sounding harsh here, but I suppose its a lesson to us all, its not worth the risk, it wouldnt have taken much to plonk it in the windscreen whether you were tired or not. Ive twice gotten parking tickets, not because I have parked illegally, but because I have forgotten to show any time once (I was max 10 minutes) and the second was because I didnt display it very clearly. So both times, I was in the wrong, even though I hadnt stayed longer then the permitted time on either. So I just accepted the fines, paid them and got on with life.
Mark, it doesnt matter whether the car was outside his house or not, the taxation rules still apply, unless its on a private driveway or private land. If it was on private land or a private driveway, then you have a case.