From ISFDB

An Imprint of the paperback division of W. H. Allen and Co. (Better known for Target books here, because of all the Doctor Who titles.)

Usual ISBN prefix 0352 - although there's oddities like "0-426-12394-8", where they kept the same ISBN as the earlier Tandem printings.

Addresses

1977: 123 King Street, London W5 9JG ^[1]
1980: 44 Hill Street, London W1X 8LB ^[2]

Sources

1. ^  The Lion Men of Mongo 1977
2. ^  The Lion Men of Mongo 1980