Modern relays are marked differently than the original ones. To convert these back to the old Lucas type which appear in the wiring diagram:
30=C1. 87=C2. 87a=C3. 87b=C4. 86=W1. 85=W2
If you don't have a wiring diagram, I'll try to explain the hook-ups. Purple wire from the inline fuse (Brown/Blue wire to control box) goes to 87, Purple/Black wire from the horn push goes to 86, Purple/Yellow wire to horns go to 30. The remaining terminals on the relayare not used.