Chrissy wrote:

I have discovered there are ants in my compost bin which surprised me
because I thought the temperature would get too high.

Ants are *everywhere* outside. If it's heating well and turned
regularly, they probably won't move in, but you're not going to stop
them foraging.

I have a massive compost heap at the bottom of the garden but this is just
one of those plastic bin things.
I've also been informed that someone put any killer powder in there a
few months ago. So, 2 questions I'd love answers to, why are there
ants, I've never had a compost before where I could see 'creepy
crawlies' in it

What, not even worms? In a 'massive heap at the bottom of the garden'
that astonishes me. Next time you're turning it, stop and look
closely...

& secondly, is the compost safe to put on my herb
garden, since it's had poison in it.

I'd be reassured that the ants have moved back in despite the poison,
but if you're worried, use it as mulch round the non-edible bits of your
garden.

I've been trying to grow organic herbs but feel this ant-killer powder
has compromised that.

Well yes, but your garden, your rules. It's not like you're going to
get inspected and certified organic.

Peter