You don't currently have a problem with horn electrics Les. The horns are wired via the auxiliary position of ignition switch (first click from off) which also controls the main radio feed. Consequently it's entirely normal for the horns not to work with ignition switch in off position.
Edit: Apologies Les just re-read your post. If the battery is good enough to start the car it should be more than good enough to operate horns. Worth trying in the auxiliary position first though to see if that works without ignition switch loads. As Jon suggests try cleaning fuse 6 contacts which also carry main radio feed.
FWIW I fitted air horns requiring a relay to reduce current through horn contacts. For simplicity I used the same fuse 6 supply though having uprated it to 20 amp. It's worked fine for seven years and I'm still on original 2018 battery. Will be renewed before coming winter though.
