Principal Contractor failure under CDM 2015 on an HRB

You are the architect and lead designer on a £4m, 8-storey residential apartment building in Manchester being constructed for a private build-to-rent client. The building is 19m to the top occupied storey and contains 24 dwellings, falling squarely within the higher-risk building definition under section 65 of the Building Safety Act 2022 and the Higher-Risk Buildings (Descriptions and Supplementary Provisions) Regulations 2023. You hold dual Principal Designer appointments: under CDM 2015 Regulation 5 and under section 31 of the Building Safety Act 2022. You completed the Gateway 2 submission successfully eight months ago. The contract is JCT Design and Build 2024 with bespoke amendments. Construction is now mid Stage 5. The Principal Contractor is a national main contractor with whom your practice has worked previously. The named site supervisor on the appointment is an experienced SMSTS holder, but on your last three monthly site visits you have found him absent from site, with day-to-day supervision delegated to a junior site manager who admits he has not had formal CDM training. The Construction Phase Plan, which you reviewed at handover, has not been updated to reflect three significant design changes issued under clause 5 of the JCT, including a revised crash deck arrangement at level 6. Two days ago a sub-contractor's operative fell approximately 1.8m from a working platform and sustained a fractured wrist. The incident was reportable under RIDDOR 2013. An HSE inspector attended the next morning, issued an improvement notice on edge protection, and has requested copies of the Construction Phase Plan, the F10 notification, and the Principal Designer's pre-construction information pack. The client has telephoned you in some alarm and asked: as architect and Principal Designer, what are your obligations now, what action must you take in respect of the Principal Contractor, and is the practice itself exposed?