I guess I could just leave it on. I used to use Llama but didn't bother installing it this time as I didn't really actually use it. And how will it know if I'm in a car or on a train?

Well my car stereo always lives in my car so that's how it knows I'm in the car. (I assume you mean to pair to something else?)

yeah, visor-mount mic/speaker innit. But it's not going to know to BT-on and pair with it without me telling it that it's in the car.

Nope, tags don't remain connected the way other wireless technologies do. It's a single tap on, and then done.
On the positive, you don't need a second tag to switch things off, you can set them as toggles rather than just power on or power off. So one tap when you get in the car, and a second when you get out.
They don't come in ones, but I could send you one to play with if you wanted. Got at least half a dozen lying unused here.

Hmmmmm. Thanks. I had the idea I'd stick it to the phone holder, not sure the toggle method would work then! If you've genuinely got spare and it's not too much of an arse, I'd take one or two and see if any spare Steam codes are of interest, I have a few Humble Bundle duplicates I think.

PM me your address and I'll send a couple your way.

Bought a pack of 5. First one is now being used to active a tasker task that turns on the lights, turns on the HTPC (which turns on the TV) then starts a TV channel