In the first instance, check that everything is in good order. As L.A.B. mentions, rotors lose magnetism. Ensure that all connections are clean and good and that you have decent earths. Clean the lighting and main switchgear contacts.
If you have a 45w main headlamp then you can upgrade to a 55/60w halogen unit (Nortons did just that for some markets with no other changes). I don't think that you can legally run more than 60w in the UK.
The charging system's ability to keep up may depend a little on your useage (presumably no indicators ?) Do you ride with the lights on and is it mostly town work ?