I can see only one marriage of a Ralph Speechley in Hunts Marriage Index - that in 1731. Yet it looks as though there might have been a second marriage c1750. These parishes are close to Northants and Cambs, so I wonder if there might have been a marriage there. There's a 1749 member submission marriage on the IGI in Whittlesey, Cambs, which is only six miles from Yaxley, so might be worth checking out.
If a child died a later child of the same parents was often given the same name. But I can't see a burial of a William 1735-57, which makes me wonder again if these are two separate couples
I can't see that Hunts FHS has transcripts of either Yaxley or Orton Longueville available, but both parish registers have been filmed by the LDs so are available to order and view from your nearest LDS Family History Centre, or at Huntingdon Records Office (as you live in Cambridge I'd check at Cambridge Records Office first)
David