It is not as clear cut as it used to be, but i believe CAT.OP.MPA.280 b) 2) is the relevant part. It states:
(2) If an in-flight fuel check shows that the expected usable fuel remaining on arrival at the destination aerodrome is less than:
(i) the required alternate fuel plus final reserve fuel, the commander shall take into account the traffic and the operational conditions prevailing at the destination aerodrome, at the destination alternate aerodrome and at any other adequate aerodrome in deciding whether to proceed to the destination aerodrome or to divert so as to perform a safe landing with not less than final reserve fuel; or
(ii) the final reserve fuel if no alternate aerodrome is required, the commander shall take appropriate action and proceed to an adequate aerodrome so as to perform a safe landing with not less than final reserve fuel.
The old regulation over here was quite sensible, but is not in force anymore, according to that you could commit to an aerodrome if it fulfilled a clear cut set of paramenters (two independent runways served by instrument approaches, vis >5km and ceiling > 2000ft).