Bug
I’ve got no idea what this is but all I saw was big tail which, to me, means it might sting. It was flying around in my bathroom. Not being an airy fairy tree hugging animal lover I simply unloaded a good 30 second blast of fly spray on it and left it for 5 minutes. Seems to have done the trick 
Anyone got any clues as to what it is [was]?
Yes. Its a bug that lives in woods and fields.
…and dies in bathrooms. But what’s it called?
i think it’s a wildebeest
Nope - pretty sure they’re large quadrupeds whereas this would be a large insect.
dead, that;s what it is
Yellow Ichneumon. Netelia testacea. A parasitic wasp which lays it eggs in the eggs or larvae of other insects. Their own larvae live inside their host, which dies when the ichneumon larvae emerge to pupate. This large species often comes to light in houses (or in this case bathrooms). It parasitises some of the larger moth larvae.
Ooh - 10/10 for finding that one out. So it’s a wasp, it deserved to have the amount of fly spray I unloaded on it then. I ****ing hate wasps.