Have a look at the double-row connector to the indicator/high beam stalk (there are 2 separate connectors, one single row, one double). Is there a brown/white wire in the semi-circular sticky-out bit in the centre of the connector. If so, unplug the connector from the stalk and connect the terminal to earth and see if the horn works. If it does, you need a stalk with the horn push on the end - someone has perhaps fitted a later stalk where the horn moved to the middle of the steering wheel.