Just to be clear, only one written representation per interested party will be allowed, even if this contains entirely different topics under relevant headings and para numbers. Is that correct?

We would expect Interested Parties (IP) to submit one Written Representation covering all the matters they wish to raise. However the Regulations do not prevent IPs from submitting multiple representations covering different topics if they so wish provided they are submitted by the deadline given in the Examination timetable.
An example of an Examination timetable can be viewed here. This shows, at Appendix A, that there is one deadline for Written Representations and then opportunities in the Examination to respond to the Examining Authority’s written questions and other parties representations. Please note we advise that clear and concise submissions add more value to the Examination.