Hi Richard,
It depends where you are going to leave it. If you are going to leave an ebike all day somewhere it doesn't matter what you do it will be stolen. As others have mentioned, portable battery angle grinders will have your security removed in second. At home in my garage I have a
Evolution 1090 Integrated Chain - 10 mm X 90 cm Sold Secure Gold
That connects to a solid ground anchor imbedded deeply in my concrete garage floor. I have 3 security cameras, security lighting and additional garage door security, and two alarms. When I take my bike out and nip into a shop I use one of these
ABUS folding locks - Locks - Bike safety and security They are not resistant to bolt cutters or nut splitters, or angle grinders, however they come with a mount that fits to your bottle cage mountings and it doesn't rattle. I've also had the Kryptonite U locks like the Forget about it, they are sturdy but rattle like hell if attached to your frame, I lost mine when it fell off doing a down hill, £90 down the drain.
I never remove the battery, but never leave my bike for more than a couple of minutes. Actually that's not entirely true. When I went for my third Covid vaccination there was a tyre fitting business across the road and I asked them if they would look after my bike while I got jabbed. They were nice lads and the manager said yes, no problem, they let me use their toilet as well when I got back. I'm not generally that dependant on other people, but a bit of respect and a bit of banter can work wonders.
Tip of the day. If you have home contents or building insurance, you can insure your bike for next to nothing. I paid £175 last year for my £100,000 contents and£2M building through my mortgage company. I declared my ebike as a £5 k asset and got charged an extra £35 for the year, that covers at home and away from home and third party liability, thank you Nationwide. Just to be sure I don't work for them LOL.