12N7 for conventional wet lead acid type or Motobat MB9U for a sealed AGM. Some owners have had problems with sealed batteries. Type " battery " into the search function at the top off the page and you will see all the previous threads on this topic.
Thanks Hendred for your help. I will buy a 12N7 battery and see if the Silk will run. I have had the Silk for 3 years and not got around to sorting it out as yet, but it sure does look good.