My understanding is you can try to obtain a class 1 at any time, it is valid for 5 years ,however if you are flying class 1 it has to be revalidated every 12 months but if you are not flying commercially you can downgrade to class 2 as long as you keep that current. If you revalidate the class 1 within 5 years with your local caa approved doc you don,t need to go back to Gatwick.
hope that helps
