won "Best British" at a fairly big show
a) If I switch the lights on with engine switched off, the headlight still shines at full power on both dip and main beam.
To be clear:-
. "engine switched off" is ignition switch 'off'?
. "headlight still shines at full power on both dip and main beam" is both filaments are on at the same time, or only one or other of the filaments is on, depending on the handlebar dipswitch setting?
. Both headlamp filaments on at the same time would be wrong normally and could be the reason for "If I then switch on the lights, the ammeter hammers over to maximum negative, really hard."
. Otoh, only one or other of the filaments 'on' depending on the handlebar dipswitch setting could be normal given certain wiring connections and switch settings:-
.. If, by "lights on with engine switched off", you mean the ignition key switch is 'off' but you can still turn on the lights with the toggle switch on top of the headlamp shell, that's normal if the wiring has been connected to the ignition switch following "Fig. H32. Wiring diagram all models from DU66246 (Home)" on page H31 of the workshop manual.
.. In this case, the Brown/White wire supplying Lighting Switch terminal 4 is connected to a 'double' spade terminal on the ignition switch and the Brown/White wire from the Ammeter is connected to the same 'double' spade terminal. As the Ammeter is connected to battery -ve by the Brown/Blue wire, simply switching on the Lighting Switch will turn on the lights.
.. However, the Lighting Switch should still have two 'on' positions - central should turn on just pilot and tail lamps, only
fully to the right should turn on either
headlamp main or
dip depending on the handlebar dipswitch position (and turn off the pilot lamp).
b) When I switch the ignition on, with the lights switched off, the ammeter flicks left (negative), further than it did before the problem started. If I then switch on the lights, the ammeter hammers over to maximum negative, really hard.
Does your bike's Ammeter have "8" or "12" at each end of the scale?
Risking telling you something you know already, the standard Ammeter wired normally is between battery -ve and the bike's 'consumers' - ignition coils and bulbs. So it shows the current being drawn from the battery at any given time - when you switch on just the ignition, the coils draw about 3-and-a-bit Amps, switching on just pilot and tail lamps draws about another Amp, switching on dip draws about another 3A (assuming standard BPF 40W dip), switching on main draws about another Amp (assuming standard BPF 50W main).
Finally here, following another recent thread, does the bike's dipswitch have the correct-colour wires (Blue, Blue/White, Blue/Red and Brown/Black) or different colours/combinations?
The battery is a yellow Motobatt, around 7 months old.
Hopefully then it isn't the battery; unfortunately, from other posts, there doesn't seem to by any rhyme or reason why some batteries last for years
but others of the same make crap out after only a short time.