Hi John440,
Yes, you have 6 months to apply for an EC. So if you don't plan to leave for UK immediately, I would suggest that you apply for EC near the time of travel.
This way, you will get more time to settle down in UK and become economicaaly active, before it's time for an extension. This particularly true if the ECO stamps a 1year visa on your passport.
Not an exhaustive or sacrosanct list, but this is what you should attach to support your EC application:-
1. HSMP approval letter.
2. All documents you sent to HO for HSMP approval.
3. Bank statements / other documents to prove your financial condition (bank statements should cover the previosu 4-6 months).
4. Payslips covering the last 4-6 months.
5. IT returns (last 3 years, if you can).
6. Proof of accommodation (letter from someone you know in UK who is ready to accommodate you, hotel reservation for a couple of weeks etc. - depends on individual circumstances).
7. If you're married and your spouse is also applying for EC as your dependent, and if you did not claim points for partner's achievement in HSMP, attach your marriage certificate for sure.
8. If you have a job offer in UK, it will sensible to include the offer letter.
9. Of course, the application form (VAF1), photographs and required fee
regards