Hi Ben, I have much the same problem over an illegitimate birth which I believe was that of my 2great grandfather, John Luffman. In Somerset militia records held at Kew he is shown as an 18 year old trumpeter, and his age is consistently shown both in subsequent marriage (Alford Lincs 25.12.1797, and his death in Alford 1821). The only baptism I have been able to locate is a strange one at Henstridge Somerset, where a Betty Meatyard baptised 2 sons, both illegitimate, John, within a week of each other in June/July 1777. The second John has the surname Luffman crossed out, which I was told by records office staff meant that the vicar had a good idea who the father was. Others say that this means the named father contested that the child was his.
No payment was made to Betty for either of these children so far as I can establish. There was only one Luffman family in the village, and they had only one unmarried son, so I have assumed that John is my 2great grandfather, in consequence I was able to take that side of my tree back a further 3 generations. However on the Meatyard side, I was kindly given a tree which took Betty's ancestry back to around 1300AD, so if my assumption is correct I have one unbroken line of 22 generations. As John was still young enough for militia service in 1814 I looked at the militia list for Alford which survives in Lincoln archive, (the original) here I encountered another problem, parts of the list which were originally written in red ink have completely worn away, and there is often nothing at all left. In these cases only a scan, I think in UV light, but stand correction, would bring up these entries, which I think shows that even original documents are not 100% reliable.