Sorry Trev , I never thought to check Heal! I suspect that the St Martins Lane is the City one by London Bridge, which makes me think this may have been a retailer's rather than a lost register mark. Pity that the other, very similar buckle with the SE overstamping this one is not the understamp! But I still feel that the SE is a lost register mark of Samuel Eaton ( first recorded mark 1736, last 1767) of Huggins Alley (later Lane)
Anyhow thanks to Silverly and Dognose we have a RESULT ! I am 99% certain this mark is that of an Edward Bills.
Now to check him out at Goldsmiths Hall ! He does not seem to have trade card anywhere, although searching for Bills in a trade card list is difficult!
There are several Edward Bills known for the period, but none are silversmiths. One, whose will was proved in 1784, was London cabinetmaker