Thanks mmm345,
Under the AIP - Alternates section (11.7)
it says as its first point under ~General (11.7.1.3) "When an aerodrome forecast is not available .... PIC must make a provision for a suitable alternate"
Then the the other sections go in to Weather conditions and Nav Aids.
So I reckon the starting point is you need a TAF, lest you need an alternate.