Ask an Property Solicitor. Get an Answer ASAP.
What you have probably got is a standard letter which covers all eventualities, good, bad, indifferent.
The reason it is been referred to the enforcement team is that whoever you wrote to doesn’t know what they were doing!
You ask whether you can challenge the decision. They haven’t actually made a decision, it’s just been referred and until you hear from the enforcement team, you don’t know whether you are going to be prosecuted or not. Only when you hear back from them would you be in a position to challenge/defend any prosecution on the basis that you talk about.
Although basic physics, water is absolutely level and therefore there only needs to be a bit of a dip in the defences a bit further along in the water will find that weakest link.
However just because there are other illegal repairs doesn’t mean they have been condoned, it just means that the person has not been challenged or indeed the Agency may not have found out about it yet.