The guardian of safe working for trainees in GP placements is provided by the single lead employer.
The contact details can be found through the SLE website: Guardian of Safe Working Hours – Shared Services (sthk.nhs.uk)
The starting point for a trainee who feels that their hours may not appropriate in some way is to raise the matter with their CS, followed by their ES and then the guardian of safe working. It would also be sensible to talk with the PDs.