Thanks for contributions - I tried the crank tightening suggestions and that seems to have worked. Mine was a definite creaking sound, not squeaking. I've just tightened up the cranks and went for a short ride - the creaking seems to have gone. I'll update if it comes back. So, re tightening the cranks, I used a large Allen key (hex key, Allen wrench), and with the bike upside down, key and crank arm at about 180 degrees, pushed (with large folded rag for cushioning) with all my strength. I had to do that a few times each side. You push hard and it tightens, but then you find you can push hard some more and it still tightens more etc.
P.S/ I just went for a slightly longer ride. Starting off without e-assist there was some creaking again. I'll just live with it for a few days probably, and see if it goes away. Otherwise I'll take the crank arms off and have a good look at them. Perhaps some water got in when I washed the bike last weekend. Would water in the crank arm connection cause creaking?