Ad Colligenda Application where there is a caveat

I need to make an application for an ad colligenda grant where there is a caveat in place. I have established that I will need a PA1A and a witness statement, but I am unsure about how/when to give notice to the caveator, whether I should be making a pre-lodgement enquiry and/or applying for leave to apply to the registrar before making the application.

Has anyone done this recently and is able to provide me with some guidance?

Also, does anyone have recent experience of turnaround time at the Probate Registry?