Thanks to all who've replied.
I've tried taking the back, revolving gate and the lens off whilst tweaking with the lever, but it stil lwon't reset. I can't see pins sticking out or anything that are causing the lever to stay stuck, so it's probably an issue inside the body. I think my best course of action might be just to get it fully serviced.
Thanks once again to all who've replied to the thread, it's much appreciated.