Many thanks for writing.
Referring to attached (initial attempt) - as I mentioned, I've become bogged down setting things up for part b. I suspect I've already come off the rails in what I have done.
So, given the connected particles are in motion they therefore have overcome Limiting Equilibrium and hence Frictional Forces (Fr3 and Fr4) both = mu * their Reaction Forces (R1 and R6) resp.
Also, given coef friction for P is mu and for Q is 2mu then Fr3 = 3g(mu) and Fr4 = 8g(mu)
However, I am concerned that I have not taken into account the vertical component of F8 = 45sin(40).
At which, I am stuck. If only these Edexcel boffins would have provided full solutions (as they do in the course collateral) I would have had a fighting chance on figuring this out. Ive done innumerable questions on connected particles but never one like this i.e. different coef of friction for connected particles. These sort of questions have tended to be truck and trailer or train and carriage type scenarios.
Apologies for not being able to have dug deeper. I hope this helps explain how bogged I am.