Use the "Authorise Client" bit online instead of the paper version. Much easier. If you are going to be filing online for the client then you'll need to have authorisation (unless you're using their own log-in). Or if you need any info from HMRC you'll need to fill one in. If you don't need either then it isn't mandatory - just makes life easier.
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Never buy black socks from a normal shop. They shaft you every time.
I recently went through a bit of a purge on 64-8's, some seem to have been lost when HMRC went totally computerised and others I think were probably never applied for. I ended up applying for authorisation (The Peasie method) to over a hundred clients. Big mistake, about a week or so later the phone never stopped ringing...'Rob I've been sent a letter from HMRC......' If you are playing catch up then stagger this! Without the 64-8 of course you can't talk to HMRC about the client so very useful, however if you are acting as the bookkeeper and not filing accounts and the client has an accountant then that person should hold the 64-8, so don't go applying and annoying the accountant/client at a time when the accountant needs to clarify something with HMRC!