O/R: on request, indeed. Translates to "give them a ring to begin with".
For getting diesel, you will have to find your field, there are some clubs that have a diesel pump, often because they operate a diesel-fuelled plane themselves. I think EBTX Verviers-Theux is one but you had better check with them. EBAW Antwerp has no mandatory handling, and must surely have Jet A1 - but their landing fees are substantially higher, of course. Could look into EBKT Kortrijk too, they have good restaurant.