I have discovered i can generate a report with my customised 'PDF style template' if i set 'Page Orientation' to 'Landscape' rather than 'Auto' or 'Portrait'.
Would be useful if someone could check if it makes a difference for them.
EDIT: hmm, if i edit the 'PDF style template' in an existing report pack (that got past the spinning wheel by using the default), i can then change to a different custom style template when in Landscape mode, and then switch to Portrait mode, and it works as it should.
If i try and switch 'PDF style templates' when in Portrait mode of an existing report pack, it doesnt have any effect, but also doesnt hang with the spinning wheel.