I asked a similar question and got a reply from MHRA Inspector G. Francis 01 July 2019:-
A: The 24 hours is to get to the Sponsor. If the Sponsor has set up a system where that notification has to go to the CRO first, then they need a robust process to ensure that the CRO passes it on immediately to the sponsor to meet that requirement.

Follow-up Q: If the Sponsor outsources almost everything (but obviously not the oversight of the CROs by the sponsor, which remains a sponsor responsibility) this means that the reception and evaluation of SAE/SUSARs will be performed by a contracted CRO. In that case I assume that the 24 hours timing is from the investigators first awareness to reporting to the CRO with SAE/SUSAR evaluation and reporting responsibilities. Is that right?

A: Yes, in that scenario we would expect the investigator to report in 24 hours to the CRO – essentially the timeline is there to make sure that the information gets quickly to those that need to report it, and of course within the legislative timelines to the Regulatory Authorities