Hi Claire,
Perhaps you have already elliminated this one but the only Berwickshire birth for a David Sl*gh I can find on S.P's free search was born 1799, Cockburnspath. Putting in the father's name as W*m, however, brings up no matches. Sometimes I have found that the informant has put in the wrong names for the parents.
I have looked at the M.I.'s for Abbey St Bathans, Ayton, Bonkyl and Preston, Parish of Bonkyl, Chirnside, Cockburnspath, Coldingham, Duns, Edrom and Eyemouth but William and Isabella are not in any of these. The old churchyard at Preston has the highest number of Sligh M.I's. I can't see a David either.
On Family search there is a marriage of William Sligh and Isoble Galbreath on 3 Apr 1796 in Ayton. The only child I can find for them was Helen Sligh born 1805 in Duns. This cross checks with S.P. Not much joy there!
I think this is going to be one that will keep you going for a long time.
Good luck,
Rae