Trouble is GP's are reluctant to change AD's and tend to persuade us to stick with them. From experience, I would say you really need to give the higher dose about 3 months. While they can sometimes make us feel good in the beginning, the real purpose of raising certain chemicals in the brain, do take longer.
In saying all that, Mat, GP's often think they know it all, but very often AD's are not their strong point. I think too many of us get caught up in months, even years, taking the same AD, but getting very little from it. In my case, I waited 13 yrs before I found something that did work and, believe me, when it works, you know about it. It's worth bearing in mind, however, if I had followed my own instinct, I would have insisted in changing them sooner. So, at the end of the day, it's your life, your recovery. If you feel you want to try something new, then insist on it. I know that's difficult when we're feeling at our most vulnerable. Maybe if you don't feel you can talk to the GP, ask the Doc to refer you to the local Psychiatrist and Mental Health Team. They are best placed to assist you with the right medication and support
Good luck