Well, perhaps you could use the smear as a good reason. tell them you had a letter reminding you to go for one ( i get one every 3 years I think.. or is it 2 ? anyway, just say you got one, and thought, you ought to this time... maybe ?
I do hope are able to move forward from this now, and not need any further treatment.
They will take another test, and a blood test and likely scan you too. then if any thing is seen still there, you'd need a small operation. It would be an E.R.P.C. if you do. which is "evacuation of the retained products of conception". sometimes needed if it did not all come out. it may have looked complete to you, but sometimes bits of the sac, or placenta etc may have been left in. if you dont have it done, it really can lead to infection which can cause future problems.
Good luck and i hope you can find time to go. Its important time really, for your health.