It is really up to you as to whether you seek a change in anti-depressant. You are aware that you are quite a way off the maximum dose of Venlafaxine ? This is 375 mg. I was happiest when someone had reached the max dose and stayed on it for 6-8 weeks, that would count as a treatment failure. You can't always do that with Venlafaxine because it has a tendency to raise blood pressure, especially at high dosage.
If Sertraline had a good effect on you I am pleased, as long it was the Sertraline. The nature of depression is to have periods of relapse sometimes after years of depression free living. I hope you didn't just experience the natural ebb and flow of depressive illness.
There is also the possibility of augmenting Venlafaxine with a second anti-depressant. The most common combination is Velafaxine and Mirtazepine. People would often opt for this when they had had some benefit from Venlafaxine but not sufficient. It isn't really possible to mix Velafaxine with Sertraline so the simplest solution is out.
But if you have faith in Sertraline that goes a long way. I would personally be making my way to the max of Venlafaxine as rapidly as possible, and then think about a change if nothing happened. But as I mentioned before it is entirely up to you, and your GP.
Last point, therapy. You haven't mentioned it as an option. In mild to moderate depression it can out perform drugs in many cases.
