Jedimistress is absolutely correct that you only have to tell them normally. Unless you are on some sort of unusual retainer arrangement accountants usually only fee for work done. Ask for an updated bill, pay it, and tell them you no longer require their services.
However, unlike jedimistress, I do think it's pretty unusual and a reasonably big step for someone to do limited company work themselves. The Companies House stuff can probably be done with some handholding provided you understand basic accounts but Corporation Tax is a more complicated issue than Income Tax and the returns require submission of full accounts with them. I'd hesitate to advise it.
Maybe you actually meant that you employ someone as a book-keeper (or pay a firm for it) and want to do that part yourself though?