I'd like one, but £1000?
As has been said before, the creatures are, in all likelihood, freshwater polychaetes (which occur [sparsely] across various isles of the Indo-Pacific, through a broad pan-tropical [and warm-temperate] belt of both New and Old World, to such outposts as the Caspian Sea, Lake Baikal, and New Zealand), thus accounting for the resemblance to a marine "bloodworm"; they likely came in as juveniles or cysts amongst your live plants.