Hi Liam,
From your e-mail, I couldn't tell if you are looking for a studentship or you intend to pay for it yourself. In the former case, I would say that there is no general rule regarding the timeliness of a PhD application. There are PhD studentships available all year around. However, it depends on your chosen topic, when and if a good opportunity will come up.
If you desire to contact a particular university/suoervisor,then... the sooner the better.