'I cannot find a burial date for Richard Slade.'

NBI has the following for the burial of Richard Slade between 1766 and 1776:-

31/7/1772 Blessed Virgin Mary, Abbas Combe, Somerset
1/11/1774 St James, Southbroom [nr Devizes], Wiltshire
22/10/1774 St Nicholas, West Pennard, Somerset
29/1/1775 St Mary, St Neots, Huntingdonshire