There is a 1795 Braddan decree for William Harper - douglas, 3ch (? thomas charles = 1 ?)joseph, thomas charles and eunice amelia harpur (children under age) mo margt;inv + sale
This was probably Mr Wm. Harper, Surveyor Generel of the Highways." as per Ochan register

There was a Harper family in Malew (Castletown) but can't see the connection with Wm

There is a marriage at Douglas St George 17 SEP 1789 Wm Harpur to Margt Cusnahan [Cosnahan ?als moore] - Margt widow;both douglas;sp licence
with Thomas Charles bapt there 17930227 (+ noted next day in Malew register) - you might also find www.manxnotebook.com/fulltext/an1847/p465.htm of interest