I have a 2012 Burgman 400 which has only just covered 5000 miles. Doesn't get out very often and hasn't been out since last year. Last outing last year, battery died whilst parked but started with a jump start and rode home, presumed knackered battery. Bought a new battery this year, installed it & went for ride. rode fine but once i got back & stopped it, battery didn't have enough power to restart. My first thought was alternator, but is it likely with such low mileage? Would love some insight before it has to go to a garage.

You need to test the battery with a voltmeter, when running you should get a reading of 14+ volts.
new batteries do need charging fully when installed, and using after that. I put a new Yuasa battery on my bike, fully charged, use for about a week, then stored for a month. Then it would not start. I did think of returning it, but stuck with it, kept charging it, and now it functions properly.

Yes you need a cheap voltmeter to make sure that the battery is charging as Dave said. You also need a decent battery charger of the kind that is 'intelligent' and will maintain the battery without overcharging. Using the charger on your new battery will greatly extend its life. From what you say there are long periods when you might not ride so you need to use the charger for about one day every month and it will be at least 5 years before you need think about buying another battery.

Regulator/Rectifier. Common on lots of bikes but especially Suzuki. The Yamaha R6 version is a common upgrade as the diodes and cooling fins are bigger. Ask victor about his.... I rewired it in the middle of Dartmoor after it burst into flames

What Lea said above.
Go here and have a little read:
http://www.electrexworld.co.uk/acatalog/Technical.html

Then click to the left at the item/s you need, ring them if you do not see what you require.
They are a UK company and try hard to help. They made a reg/rec for an old bike i had some time ago as no parts available any longer.
Lyn.