Derek, A bit of a hornet's nest this one. When working well 6 volt systems are fine,but they seem to be at the limit of many tolerances and it would appear that it takes very little to cause system to malfunction. The conversion that I made to 12 volts has given no trouble at all in the last five years.
I converted my RP to 12 volts as I needed better lighting when driving to & from work,A VAST IMPROVEMENT. Better starting,acceleration under load,charge rate,can use LED bulbs and other accessories such as GPS CD's etc.
The conversion was easy.The existing 6 volt dynamo, modified to two brush operation, will easily produce the 12 volts required and the voltage control is by a electronic unit under the dash out of sight.A change of bulbs,coil,battery and you're done.Not a difficult job,control unit purchased from Dave Lindsley & cost in 2010 was £106.& includes instructions on dynamo mods, again very easy.A lot cheaper than fitting non standard looking parts.The only visual difference is the dynamo brush cover which I changed for some fine wire mesh to aid cooling. The only item that doesn't operate is the fuel gauge which didn't work on 6v either, I'm currently looking for a gauge to replace the dash instrument to match up with a modified "Mini" in tank sender,but will always rely on the dipstick method.
I wouldn't even think of recommending 12 volt conversion (even though it has benefits :-) )but if you do it, these cheap thingys are a great way to power the 6 volt wiper motor and fuel gauge..http://www.ebay.co.uk/itm/DC-DC-Power-Converter-Regulator-Module-Step-Down-Adapter-12V-24V-to-6V-5A-/131243097597?epid=929908673&hash=item1e8eb2bdfd:g:98oAAOSwwpdW6Tal