FL800, hi there.
Well, it all depends on the incursion. However, as a general rule the commander may decide whatever he/she deems appropriate including a decision to come back and land provided the commander assesses the landing to be completed safely. That includes weather, traffic, ATC, etc.
Are you only required by JAR to declare a MAYDAY if you are going to land with less than final reserve?
JAR requires one to declare emergency when
ACTUAL fuel on board is equal to final reserve. Check your part A since the company may lay down more restrictive rules.
Once again in order to make the life easier and decision making more convenient JAR basically says do whatever you want but make sure you have final reserve upon touch down. That's all.
Cheers
P.S. It must be emphasized that the lawmaker expects one to have at least alternate+final reserve at IAF at the destination in regular case unless the commander has made the decision to continue to the destination while en route (as described above after assessing the whole thing) despite the fact that the fuel over IAF is predicted to be less than final+alternate. It's entirely commander's responsibility anyway that's why take some extra and don't get yourself into trouble.