Thanks for all the replies so far.
It's interesting that they've withdrawn the 50% discount, and I think I do agree with H C Anderson that they may well be able to without invalidating the PCN, however I strongly believe that by stating that ""We cannot accept verbal challenges in person or on the telephone." they are acting contrary to the Equality Act 2010 and as such I believe that will make any PCN on which they state this unlawful.
This isn't the only defence against the PCN (which is actually for a neighbour of mine which is why I can't post all the details - they are not mine to post.) I am 100% certain of the other grounds of the challenge against the PCN but having seen this new wording I wanted particular advice on that.
As anyone who was on Pepipoo may remember I had a regular and particular problem with Havering attaching PCN's to my car when the car was parked entirely legally. I have won EVERY challenge, representation and tribunal hearing that I have undertaken on this issue and it seemed in the last 18 months as if Havering had finally realised that they had no reason to issue these PCN's. All I can assume is that there is a new Enforcement Officer who is unaware that our private car park is, in fact, a private car park and thus issued a PCN to my neighbour.
I shall, of course, keep this site updated with their response.
Once again, many thanks to all.