This overprint is from the firm Francis Heathcote Wenman & Co.
At the time the overprint is believed to have been in use they were operating as Ship and Insurance Brokers trading from:
The firm however had various names and locations during its lifetime:
1865Ė67 Francis Heathcote Wenman of 19
Bishopgate St. London E.C.
This was an unofficial overprint, privately printed in black, reading upwards.
The overprint is believed to have been used from around 1868 (as the only known copy is on a 1d lake-red plate 120 which was put to press on 8th September 1868). This also nicely ties in with the overprint and the trading name of the company at this time.
1868. Unofficial overprint type ?? (unlisted) in black.
Vertical overprint, reading upwards.
PP? (unlisted) 1d lake-red (SG43 or G1)