There is an excellent searchable online database administered by the Universities of Cape Town and the Western Cape, in partnership with the Cape Town Archives Repository and the National Archives at The Hague, whose goal is “to make a complete transcription of certain papers from deceased estates administered by the Orphan Chamber at the Cape between about 1690 and 1840, and a catalogue of associated documents”:
The Orphan Chamber was established in 1673 under Dutch rule and continued during the British period under the jurisdiction of the High Court.
There were no CUMPSTON entries but it worth just browsing the entries and the 3 maps in the front page are excellent.
<http://www.tanap.net/content/activities/documents/Orphan_Chamber-Cape_of_Good_Hope/index.htm>