Now that I’m a fair weather rider I only really need to insure the bike for six months of the year. Is there anywhere that you can buy insurance for six months or is it 12 months only.
No Joby, its 12 months or nothing. Man up and get out a bit more. 
ebike insurance does monthly one so buy six months month by month and you sorted
And six months will cost almost as much as a year long policy.
Bite the bullet, take the year policy. At least your bikes covered against whatever, even if it’s not in use, and your earning NCD.