Here's the info you're looking for (accessible at
http://www.delta.com/planning_reserv...on/index.jsp):
National GHANA (GH) /Residence CANADA (CA)
Transit UNITED KINGDOM (GREAT BRITAIN) (GB)
Destination GHANA (GH)
ALSO CHECK DESTINATION INFORMATION BELOW
UNITED KINGDOM (GREAT BRITAIN) (GB
1. If holding Permanent Resident Card issued on or after
June 28, 2002:
visa not required if:
- continuing by same/first connecting aircraft to a third
country within 24 hours; and
- holding confirmed reservations; and
- holding documents required for next destination.
Passenger may leave the airport transit area, at the
discretion of Immigration authorities, for e.g. a nightstop
in town or if transfer between different airports
applicable, provided holding confirmed onward reservations
and correct documentation for next destination.
...
Bolding mine. I don't really know what that means in your case. I suppose a visa would be advisable if you still have time, but there's a chance it wouldn't be necessary.