This is a hard question, and one that has sparked a few heated conversations amongst my colleagues.
If Hester ever did make Hanoverian pattern cutlery, it would have certainly been during the early 1760’s, and therefore this would match up with the date. However, in David Shures book on Hester there is no mention of her doing so, and there are comments made as of the validity of her early mark. (It wasn’t that long ago that no dealer would accept any silverware marked before 1774 as authentic Hester Bateman, but nowadays we all accept her early work, and regard the 1761 mark as her first mark.) Actually as a side note to this, the earliest of her marks is just slightly different from her later marks, the B of the 1761 mark is not closed fully at the bottom, so check to make sure this is the case.
The best thing to do is take a look at the spoon, and check the marks for yourself. Look for solder lines around the marks, and check that the marks are deep enough and look in keeping with the rest of the spoon (i.e not too big or small). From this period the spoon should be stem marked, so again check this.
Being from 1762 it could have been Old English pattern originally and modified to Hanoverian, but usually it is Hanoverian that is modified to Old English. (this is because Old English became fashionable after Hanoverian and many owners of Hanoverian simply had the pieces modified to reflect the new taste for Old English).
It is very unlikely that anyone transposed the marks from another piece of silver, as the other piece of silver would probably have been more valuable than a spoon. The marks could have been completely faked, but then why fake a spoon??? Had it been a fork I would have been much more suspicious as Hester Bateman forks are much rarer than spoons and might be worth faking.
You do occasionally see fire staining on pieces of flatware after reshaping, so i would not think that this was a problem.
It is hard for me to tell you anything more without seeing the spoon i’m afraid.