It's debatabable if Bosworth took place in the case of Anne's survival. King Richard OTL was despairing after the death of his wife and son, and rashly committed to battle before he was ready. ITTL he very well could decide to wait for the substantial reinforcements from the North, where his powerbase was. That would also serve to force the Stanleys to choose their side. They can't linger too long with their army before the King questions their loyalty and moves on them, or at least takes steps to defend agaist their treachery. So, a more confident King Richard does away with the usurper Tudor, and we'd see a restored Plantagenet monarchy instead of, in the end, an obese, syphillitic franco-welsh hanswurst without a decent claim to the throne robbing the church to pay for his extravagancies.