My first thought was a 'K' or an 'L', but there weren't any matching names in the 1841 census.
Also, what there is of the initial letter is more or less identical to the first part of the 'H' in Haydock, so I wondered if the pen was low on ink at that point. Or if the informant had signed with a lower case 'h', might the registrar have decided to reproduce it in this copy?
I know none of this is conclusive and it relies on speculation, but on balance of probabilities I'd still go for Heyes.
Incidentally, the printed wording indicates that this certificate came from the GRO. If it was felt important enough, a copy taken from the original register could be obtained from the local register office, and this would have either a photocopy of the original entry with Mary ?'s signature, or a modern interpretation of it.
