@TMOCONTRACTLAW to answer your PM, you have the following issue: TfL will only freeze the PCN at the discounted amount if they receive notice that an appeal has been listed from the tribunal while the discount is still on offer. The NoR is dated 18 July so even if you'd appealed a couple of days ago, it would have been too late as it takes the tribunal around 5 to 7 business days to process the form and notify TFL.
However there is a simple workaround: if you pay the PCN at the 50% rate and then appeal anyway, TFL have not (in our experience to date) ever demanded the other £80 even if the appeal is refused, but obviously if the appeal is won you TFL will be ordered to refund the £80 payment.
So in short my advice is to pay the £80 now and then file an appeal anyway, as you effectively get a risk-free appeal.