Wow, thanks for the replies. :)
Shame about the damage on the top left corner, but I'm still very happy. I believe I bought it a few months ago from a 5c box at a stamp exhibition, and I found it now while sorting through the stamps country-by-country. No idea how a trial overprint ended up in a dealer's junk box...

I'm posting a transcript of the Spink auction description here (so it will be visible on Google):
Quote:
Auction: 1026 – British Empire Stamps & Covers from the Vestey Collection
Lot: 604
St. Helena 1947 (29 Apr.) Royal Visit trial overprints presented by Captain Clarke to his brother K. H. Clarke, the acting Governor, comprising 1938-44 1/2d. and 1 1/2d. each overprinted "royal visit/29th april,/1947.", 1d., 2d. and 3d. grey each overprinted "first/royal visit/29th april/1947." and 1 1/2d. overprinted "royal visit/29th april/1947", fresh mint with all but the first marginal. A very rare and unusual group. Photo Estimate £400-500
Note: These stamps were prepared in commemoration of the visit of Princess Elizabeth and the Royal Family aboard HMS "Vanguard" during the Royal Tour to South Africa provenance Sotheby's, September 1984
Sold for £950
It appears that each of these may exist with different overprints. The 1d in the Spink lot includes "FIRST", while mine does not. The lot also contained two 1½d stamps, apparently with minor differences in the text (the comma after "APRIL" and full stop after "1947").
According to the World Statesmen website (
http://www.worldstatesmen.org/Saint_Helena.htm ), at the time the Acting Governor was Frank Edward Gilpin, who was acting for William Bain Gray. Only two Clarkes are mentioned: a Robert Clarke who was Acting Governor in 1850-51, and a Maurice Campbell Clarke who was Resident magistrate (managers of the Cable and Wireless Ltd.) from 1944-47. Maybe this could be the Captain Clarke who was involved in the overprints?