A research mission for you - Cost of making a phone call in 1976

[Hibbyradge]

Can anyone help me find out how much it cost to make a call from a BT phone box in 1976, please?

I definitely remember using 2 pence pieces to make calls at some time, but I'm not sure if the cost had risen by 1976.

Thanks.

[HUTCHYHIBBY]

The GPO ran the phones in Britain until the early 80s, might be worth contacting The Post Office.