I have just been away for two weeks, and the battery did not go flat. This afternoon I started the Z3 up and it kicked into life first time. I have been using an uprated windscreen solar trickle charger, as per:
https://www.amazon.co.uk/AA-50601146.../dp/B00AC1LLQY
note: this is a 2.4W cell. My forst one was 1 point something and proved to be useless. The current one is either 5 or 6 watts. The extra £10-£20 is worth it.
I took the car for a vigorous spin to charge the battery up. Everything as expected, other than a peculiar high frequency reedy noise at speeds (let's say) around the legal limit.
I pulled over, opened the bonnet and had a poke around. Nothing obviously amiss. However, seeing as it is autumn, and we have an abundance of chestnut and beech trees near us, there are plenty of leaves around, and one (of the many trapped under the bonnet) had become caught under the windscreen wiper. Once everything was cleared, the noise seems to have disappeared. I'll have to give it another spirited test tomorrow.