Have you considered trying Albendazole and Ivermectin? I thought I had a bad threadworm infection but it turned out to be larger roundworm too (ascaris). I'm assuming you are in the UK as you mention Ovex? I'm based in the UK and have ordered Albendazole online and am using an Ivermectin-based horse wormer. You might want to check out ICUs Albendazole protocol and/or Albendazole + Ivermectin.
It can take a long while to get rid of parsites. I've been on the Albendazole protocol for 4 months now. Much better but still got a way to go. I've just added in the Ivermectin and that seems to be helping.
What about your diet? It's essential that you follow a low carb/sugar free diet because the Sugar is what feeds them. I eat less than 60g carbs a day. It's hard at first but you get used to it.
This isn't really the place to discuss drug options though. If that's the route you want to take then check out the Parasites: Drugs forum on here. Hang in there, you will beat them! I never thought it possible to get my health back to this state but I have and it's getting better all time.
(Oh yeah, and doctors are clueless when it comes to parsites btw...)