The horn problem is easily sorted, Ebay-Stebel Nautilus or FIAMM twin electric under £20. Some spade terminals, two piggy back spades, suitable 15A cable, 20A fuse holder, couple of Y connectors. Most horns are supplied with a relay.
Slip off the front grill, to allow access to the front of the rad, where the weedy original lives. There are a couple of places the Stebell will bolt, the twin FIAMMs will need mounting on a bar and that bolted in front of the rad, there is quite a lot of space here.
Crimp or solder the piggy back spades to cable, remove the original connector to the horn and fit the piggy back's, put the original connector onto the piggy back connectors, ie doubling up the connection.
Fit spades on the cable ends and fit to 85 and 86 relay connections. When the horn is actuated this will power the relay.
Now run cables from the horn or horns (if using twin horns double up the connections) one to terminal 87 and the other to earth (-negative on battery) use the Y connector you can loosen one of the nuts by the terminals and slip it under.
Make sure you have connected to the right terminals on the relay.
Now connect a cable on to terminal 30 via a 20A fuse and on to the battery positive + use the other Y connector.
Hold your ears and press the horn, problem solved.
If you run the cables back to the battery area the relay can be zip tied onto one of the big cables here.
If you want further information or want a drive to the Romford area I can advise or show you how, there is little difference in the rad area between models.
If the photos section is easy to post in I can do some.
Phil