Heat and plenty of it. either buy a blow torch, or if your other half isn't in put it over the gas ring on the cooker, and get it as hot as you can for about 15 minutes and let it cool. This may or may not knacker any brazing that's round there though, but it'll certainly free things that penetrant sprays won't. Incidentally, WD40 is a bit rubbish in this respect, try to track down some 'plus gas' spray, it's a good bit better, but still not as effective as a bit of heat.