Accountant Letter for ILR application from Tier 1 (Exceptional Talent)

Hello,

I've been in the UK now for almost 5 years under the 'exceptional promise' track of the Tier 1 (exceptional talent) Visa. In a month I should be eligible to apply for an ILR. I'm self employed and I have two questions relating to the accountant letter that is required for my ILR application. On the government website the letter is described as:

'A letter from your accountant (confirming the amount you have earned): This letter should be
on headed paper and confirm the gross and net pay for the period to be assessed. The letter
should give a breakdown of salary, dividends, profits, tax credits and dates of net payments
earned. If your earnings are a share of the net profit of the company, the letter should also
explain this. All accountants must be either fully qualified chartered accountants or certified
accountants who are members of a registered body in the UK.'

My two questions relating to this are as follows:

1- What period does the accountant letter need to cover? Is the last tax year that I submitted a tax return for enough? Or does it have to be the entire 5 years (or 4 years since the deadline for paying the last tax year hasn't arrived yet)?

2- Does that 'UK - Chartered Institute of Taxation' count as a registered body in the UK? My accountants are a big firm with chartered and certified accountants, but I'm not sure about their registration.

I hope someone would be able to help me with these.

Thanks!

This has been asked before: indefinite-leave-to-remain/letter-from- ... l#p2005905