CDM Principal Designer: fatality from a design risk, HSE investigation, and conduct duty

You are the architect and CDM Principal Designer on a £4 million school refurbishment under JCT SBC 2024. You were appointed at Stage 2. The works include a new single-storey teaching block and refurbishment of an existing roof, where the design adds three new rooflights to bring daylight into a previously dark internal corridor. Three months into the construction phase, a sub-contractor roofer carrying out routine maintenance on the new roof fell through one of the newly installed rooflights. He suffered fatal injuries. The HSE has opened an investigation under the Reporting of Injuries, Diseases and Dangerous Occurrences Regulations 2013 and has issued an FOD letter requesting documents. Your design risk register, last formally updated at Stage 4, includes a generic note that "skylights and rooflights present a fall hazard and should be specified with protective grilles or class B non-fragile glazing to ACR(M)001 where regular roof access is anticipated". The note does not specifically address the fact that, as built, no protective grilles were installed on the three new rooflights and the rooflight specification did not call for class B non-fragile glazing. The Principal Contractor's Construction Phase Plan references your design risk register but does not record any added control measures for the rooflights during construction or following completion. The client (the local authority) has written to you saying they intend to terminate your appointment with immediate effect and may pursue you for failing to design out the hazard. The HSE inspector has telephoned and asked you to attend a Section 20 interview under caution. Your PII broker is on holiday. You hold £10 million aggregate PII with a £25,000 each-and-every-claim excess. The deceased's family has instructed solicitors.