I haven't had any weight issues with lamotrigine (lamictal) either, and I understand how important it is to you as I also had an ED in the past which still on occasion rears it's ugly little head. Lamotrigine is fab, ime, as I've said before! I wouldn't say that it will cancel out the effects of seroquel, as until your off it completely you'll still have the drug in your body, and thus enjoy [hhhmm] the side effects. Once you are solely on lamotrigine though, you'll be able to lose the weight you've gained just like anyone else. This was the case with someone I know, who's lost 3 stone and counting from a combination of having their meds changed and sensible eating/exercise.
Are you doing the slow titration up to a therapeutic dose of lamotrigine? It seemed to take ages for me, but it's well worth it. I haven't had any issues, apart from shakey hands. PITA on occasion, but better than being fat and/or mad!
I take generic lamotrigine and branded lamictal, depending on what the pharmacy has in stock, and there is honestly no difference in terms of therapeutic effects. I was told the reason for this is that at my dosage, branded ones cost the NHS about £90. Generics cost about £5! [hshock]
The only difference in my experience is that branded lamictal is dead easy to swallow- I think it has a better coating. Generic lamotrigine, especially the diamond shaped 100mg tablets dissolve in your mouth faster, and get kinda stuck in your throat if your not careful. Take it from me, that is not a pleasant taste experience. Not as bad as the time I got artificial cherry flavoured soluble ones mind...