That seems to have done the trick!

Donation Amount PY:=
SUMX (
VALUES ('Event'[Event Year]),
CALCULATE (
[Donation Amount],
ALL ( 'Date'),
FILTER (
ALL ( 'Event'[Event Year] ),
VALUE('Event'[Event Year] )
= EARLIER ( 'Event'[Event Year]) -1
)
)
)

I can't say I totally understand what it's doing, I have some reading to do 🙂

Thanks for your help, much appreciated.