I agree with Squiffy and think you need to think about two sides to this: the legal and the commercial. Commercially, you need to be honest with yourself about what is (and isn't) necessary to succeed in your field and what you want long term (an exit, to stay where you are, to bide your time and move to another company...)
On the legal side, I think it is concerning that your boss is using this as an opportunity to performance manage you, and suggest you pull them up on this. My response would be something like:
"I am committed to my job and, of course, to delivering a high standard of client service. However, I am becoming concerned that what I thought was a straightforward discussion regarding my return from maternity leave - to the same job and on the same hours as I had before I left - seems to have turned into a performance warning.
Before I went on maternity leave, I was working on a 7:30 to 5pm basis for 18 months. During that time I [then explain details of your performance, appraisals and bonus]. There was never any discussion of my hours being a problem or my performance being unsatisfactory before I went off on maternity leave. Since I have sought to return from maternity leave, I have been told that you thought that the arrangments never worked and am now being told that my performance needs to improve (as an aside, we also didn't specifically discuss impact on client portfolios during our meeting). It really isn't clear to me how I can be getting strong appraisals and bonuses before going off, and return to be told that there is, and always was, a problem with performance.
You are also now telling me that, because of these performance concerns, you need to change my working hours. Surely this is a disicplinary sanction against me and something that would come at the end of a performance management process and targets, not something you would simply impose on a maternity returner straight out of the gate? If there are issues with my performance, I am happy to understand those and work on them to improve, but since these have come out of the blue for me, I think it is only fair that I return to work on my current terms and conditions and, if there are performance issues, we work on them from there, when I have a genuine opportunity to understand and address them. I do not think that changing my terms and conditions is a fair first step.
I would be happy to meet with you again to discuss this further."
Or something like that...