I found the source of my leak - water coming up through my windscreen washer pump and leaking straight out of the body - failed shaft seal
I will go at taking it apart and see if I can fix it. It's branded VDO and it looks to be at least €30 for a VDO replacement.
EDIT: I'm wondering if there's anything in that Lidl washer fluid I use that might be damaging for the seal.
Here's what's inside the pump housing including the seal on that motor shaft -
