AIP Para 73.2.1
Except when operating an aircraft under the VFR by day within 50NM of the point of departure, the pilot in command must provide for a suitable alternate aerodrome when arrival at the destination
will be during the currency of, or up to 30 minutes prior to the fore-
cast commencement of, the following weather conditions:
And then it goes on blah blah blah.....
The book does not state that this is only a planning requirement and not an airborne need. It simply says "the pilot in command MUST (my bold
) provide for a suitable alternate aerodrome when the arrival at the destination...."
The fact is, if you don't have the fuel to hold an ALT you must do something about it, ie, divert for fuel, select another primary landing point or if these options are not do-able, then declare min fuel and receive priority.
Riddle me this
When you receive an updated forecast that has either INTER or TEMPO, do you just ignore it because "you're already airborne" or do you do something about it - divert, 'make fuel', declare MIN FUEL. Of course you don't just blunder on... do you?
For those in the 'no need to hold an alternate' camp, I'm open to looking at your AIP references and looking at how you come to your conclusion; as we all know, AIP isn't always the clearest document to read!