Anyway let me know how you get on. If you're VAT registered it will generate all the VAT returns between then and now. You can just click on those VAT returns and on the right you'll see "mark as paid", that will close the return. If you're migrating accounting products you can import bank statements too.
You're right that would be hard to believe. Invoice numbers increment by 1 you can see here. http://recordit.co/j7t4oiwU0s So if you had Inv No: DJDH23 the next one would be DJDH24 If you had 100, next would be 101 and so on.