BMC (British Motor Corporation) and Leyland were merged to form British Leyland. Leyland were one of the flagship British Truck manufacturers, buut were starved of investment by a highly bureaucratic cabal that owned marques like Jaguar, Rover, Austin, Morris and Triumph. There was a lot of arrogance and inefficiency with too many heads, no rationalisation and a lot of asset stripping and payoffs. It was also the 70's when Unions were at the height of their power and the Conservatives were on the rise. With constant disagreement, strikes, double dealing and general mismanagement, a lot of companies went to the wall. Triumph, Morris, Austin and Leyland were the worst hit, with Jaguar and Rover just about surviving into the new millennium. The whole British Leyland sh*t show effectively crucified the British Auto and Commercial industries, something the UK still hasn't recovered from (thanks largely to Thatcher's greedy and rampant asset stripping).
You also might want to check the Rootes Group, BMH (British Motor Holdings) and the Leyland Truck and Bus Divisions too see how bad the carnage was and where a lot of issues began (back in the 60's). Chrysler will also get a mention in this.
On a related note Check out the take overs of AEC, Albion, Austin, Guy and Morris by Leyland to realise even more of what was lost. and why we need strong anti-monopoly legislation.