Thanks for the reply, I did look at the Henry Lias options as the first set of initials is most likely HL. However I am struggling to interpret the 2nd letter in the other initials as being an L, it looks to me more like an S.
I agree wholeheartedly re the cartouche. I am beginning to wonder if the second initials are an amphersand and S, which could mean & Son. Maybe Lias decided not to use this and opted for the double HL instead. Also the way it as been defaced is quite professional, they have deliberately avoided damaging the cartouche shape.
There is certainly a resemblance to "HL" over "&S". However I think that the resemblance is just due to the way that the defacing has been done. I can't, off the top of my head, think of any other marks of this period which have exactly this shape so I would definitely go for "HL" over "HL".
If that is so then here is a possible explanation. Originally makers' marks had to consist of the initials of the name of the silversmith so HL over &S may not have been an approved form. If Lias & Son decided that they wanted a mark of that sort, produced the punch and then found that the Assay Office refused to register it they would have had to deface any instances already used. This is pure speculation of course....