there are plumbing tools that you can thread through the water tubes to reach the nut.
Do you mean this?
Show Image
(http://itslondon.s3.amazonaws.com/p/alt/xxl/ARMDSPSETJD1_1.jpg)
It is called Jaw Dropper Wrench. I believe only Armeg Ltd. makes this wrench and it's pricey.
Thanks a lot for the suggestion. Yeah, I've never seen it online before. A bit too specialized - take too much time and money to get it into my hands.
Fohat has hit upon something that probably is what I'm looking for. A box spanner/ tubular nut. Or else a sink/ pipe wrench. The problem is that sources in English are split between US and UK English and I'm using the UK English names. This being Singapore, we use a mix of either. And then most workers are Chinese and pretty unprofessional since we don't have a professional culture for most manual work. It's quite a mess trying to find the correct tool.