I’ve no experience of the trap you describe, but if they are used to kill hedgehogs I imagine they are extremely dangerous to hedgehogs.
Are you sure that the rats haven’t been there all along, and you’ve simply not seen them before? Rats are pretty ubiquitous in this country, and likely to be in gardens everywhere. There have been other chats on this forum about how to stop rats eating food set out for hedgehogs, and such a thing seems extremely difficult if not impossible. I’d be very wary of using any sort of poison or trap, as they would also kill hedgehogs, as well as the poisoned rats also poisoning predators and scavengers, including hedgehogs, who will eat carrion. I’m not sure that there is any form of trap you can use to kill rats that is not also dangerous to hedgehogs.
