Seems to me there's a huge difference between those little books some of us put together lovingly over time, and can get possibly get cues from if necessary--and a manufactured folkie hymnal--which is in fact used as such--complete with copies handed out.   So all you in the UK, no matter what books anybody might use, are far better off than us in the US with Rise Up Singing--aka the Blue Book of Death (to singarounds).

There may be all sorts of crutches. But only one crutch is a deadly danger to a good session: RUS.

In fact a RUS session is far worse than a real hymn sing would be.   At least a real hymnbook often has harmonies which have been proven to work. And the leader would be likely to have an actual idea of how the melody goes--something you cannot assume with RUS.