To everybody that responded to my question.Just an update re my battery.I've now taken to turning the battery on when charging it as recommended by minexplorer.I've taken the bike out three evenings in succession,and covered an identical route on all three occasions.First evening the battery cut out 3 miles from home.Second night it cut out 2 miles from home,and on the third night it got me back home with some power to spare
.So it seems to be getting back to where it was originally and must have been a case of the cells being out of balance as a result of my not charging it correctly.Would have been usefull if the supplier had sent some instructions with the battery in the first place but hey ho.Many thanks to everyone for your usefull advice.