According to one enterprising journalist (I forget which paper) who actually phoned the TV station in Nigeria, a shipping container with three tons of material was returned to the BBC at least three years ago.
It's apparently taken them this long to make any kind of official announcement so that the relevant material can be restored to as near broadcast standard as is possible. Rumours of missing episodes being returned have been circulating since at least April/May.
If memory serves, there are three B&W episodes of "Dad's Army" missing from the archives, most of the early "Steptoe & Son", and quite a few of "Hancock's Half Hour".