It's a minor error that is usually over looked.
It is the director(s) decision when to declare a dividend and has no bearing on the company accountant, so should be signed by directors. There is a clause if the accountant signs the dividend vouchers, then it is considered to be a company that is managed by the accountant.This can be found if you search companieshouse.gov.uk.
Invoices, are raised by the employee / director for his / her services to a client, and should be signed by the director / company secretary and not the accountant.
Here is a post of list of documents I submitted back in 2012,
http://www.immigrationboards.com/uk-tie ... ml#p654045