The standard alternator upgrade for most British bikes is the Lucas RM24 three phase alternator, or just the RM24 stator if your rotor still has plenty of magetism?
Originally the RM24 system had a Lucas three-phase rectifier and voltage controlled by twin Zeners.
These days a modern three phase regulator/rectifier control box is normally preferred, such as a Podtronic, or Boyer Powerbox.
A complete 210W upgrade kit (not made by Lucas) should be available from Sparx (Tri-Cor England) or from certain British parts suppliers worldwide, I think:
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 ?