A new revised guidance note for judges has been published by some other judges: Joint Presidential Guidance 2019 No 1: Permission to appeal to UTIAC.
Paragraph 31 seems wrong to me, or at least inexactly phrased:
If a FtT judge considering an application for permission to appeal is in doubt whether there is an arguable error of law, the default position is that leave should be refused. The application can always be renewed.
If there is doubt, surely it is arguable? It looks like one rule for First-tier judges and another for judges of the Upper Tribunal.
The guidance is primarily for judges rather than advocates, but it is worth a read if you want some insight into judicial mindset and the go-to procedural cases.