It is my understanding you are eligible to apply for a Tier 4 visa that will cover your Pre-sessional English course and your PhD, but please confirm this with International Admissions, admint@herts.ac.uk If you can only apply for a visa to cover pre-sessional English, then you should apply for Tier 4, this can be extended in the UK for your follow on course, as long as the course start date is no more than 28 days after your visa expires.