Danny
A Class 1 medical is valid for one year until, as Splat said, you reach 40 years old. The initial issue of a Class 1 can only be done by a select few AMEs (Aircrew Medical Examiners), a list of which is held by the CAA Medical Dept. The initial issue will cost you £400; annual renewals (which can be done by a larger number of AMEs, spread all over the country) are somewhat less.