[Caecilius[_2_]]

Following on from an earlier post about a pressure reducing valve on a
megaflo cylinder, I've decided to remove the cartridge and clean out
the filter.

Guess what - it won't unscrew, and seems to be stuck fast with
limescale. Here's a pic:

http://i1021.photobucket.com/albums/...psfa4e63e1.jpg

So the question is: what's the best way to tackle this? I assume
plus-gas and WD40 are not a good idea because this is connected to the
potable water supply. Would it be worth using some sort of descaling
cleaner to remove the limescale, or wouldn't that get down into the
threads?

I'm using a 12" adjustable spanner, so one option is to use a longer
one - like a big set of stilsons, but I'm worried about putting too
much stress on the surrounding pipework.