Hi Diana. In those days, there probably would not have been civil engineering companies working in Nigeria. Most likely all civil engineers worked for the government (UK) in the Ministry of Works or Public Works Departments of the Colonies. They handled roads, water supply, sewerage systems, government buildings etc. The good friend of mine who worked in Nigeria in the 1950s with the PWD and again in the 1970s with the same engineering consulting firm as myself, died many years ago so that's a brickwall, unfortunately. Your relative may have been contracted by what was, or became, known as Crown Agents: I can't recall their original title if indeed there was one. Googling it, it says they were founded in 1833 as a UK Statutory Public Corp. Their full title is The Crown Agents for Oversea Governments and Administrations Ltd. They have a website
http://www.crownagents.com and according to that their contacts are: The Crown Agents for Oversea Governments and Administrations Ltd. Tel: +44 (0) 20 8643 3311 Fax: +44 (0) 20 8643 8232
Crown Agents, St. Nicholas House, St. Nicholas Rd, Sutton, Surrey, SM1 1EL, United Kingdom. It may all be wrong and he didn't work for them but in the 1950s it was all government employment for the colonies. If you contact them, be persistent because for them to do tracing of your Denis will mean unproducitve 'work' for them. Give it a go and good luck.
Best