Most people do write an applicant letter--if you feel more comfortable doing it, do it. But it definitely should only be a few paragraphs long, if there's anything you might need to clarify/change from your online application, etc, etc. It's completely up to you what you choose to do.
I think at this rate, you're looking for someone to tell you everything will be 100% okay--you've asked these things before, and people have answered you. The answers aren't going to change much from what they were beforehand, so all you can do is submit what you have since it seems to be such a hardship for your husband to open a simple bank account (bad credit rating or no, they hand checking accounts out like candy at some banks). If you and him aren't going to do the things that will simplify your application, then you can only submit what you have--most everyone on here has experience submitting bank accounts so for us to tell you doing otherwise will be 100% okay isn't really in our experience to say.
edit: here in the US I have a tarnished credit rating due to defaulting on student loans for a *long* time, and I was granted a very basic checking account from Washington Mutual (and I'm sure I could of gotten a savings account if I had wanted one). I know experiences will vary from bank to bank and country to country, but I can't see any feasible reason why a bank wouldn't give your husband a basic checking account--they're not set to lose anything by giving him one.
It seems as if there's 'basic bank accounts' in the UK set up for people who have bad credit ratings and who might have problems--they don't give you a checkbook or overdraft protection, but they give you a cash card and ways to set up direct salary debits and automatic withdrawls:
http://personalbudgeting.suite101.com/article.cf/what_does_a_basic_bank_account_offerhttp://www.moneymadeclear.fsa.gov.uk/HTML/en/products/bank_accounts/basic_bank_accounts_popup.htm
