How's it going in Perpignan? Did you have a bad year with velutina?Maybe I've missed something here, but come spring why would you not want to trap mated queens who are looking to set up home?
Doesn't it strike you as odd that with all of the espoused spring trapping going on the situation is only getting worse? Doesn't it also strike you as odd that you've seen a fall in numbers of indigenous species?
The situation you have is in meaningful part down to spring trapping. It hasn't worked and isn't working. That should give you a little inkling to set aside your skepticism of elainemary and the NBU. By all means continue propagating your problem in your own locale but don't equate what you have with where we are in the UK with velutina. If spring trapping is introduced en-masse in the UK it will I fear have the opposite effect just as it has done in France.
My best advice to you Richard is to put away your spring traps and instead use FC which will definitely specifically target velutina and will encourage indigenous vespine species to recover and compete with velutina again. Use FC religiously in spring and summer before sexuals are produced until your local population of velutina collapses which it will if you apply FC correctly.