Useful extracts from some Urban Dictionary or other; for no obvious reason I wasn't looking for a term to describe such a person but then I stumbled upon it, suddenly everything made sense:
Fun Sponge (or funsponge if you want it to remain a single word).
"This word is used when a person sucks up all the fun out of a room and makes the atmosphere really boring."
Like the way a normal sponge sucks up liquid.
Maybe, just maybe, if I keep trying I could become one myself or better still avoid that fate altogether. RIP self awareness and hello old age!