Sounds like there is no immediate rush so I would apply few drops of the penetrating oil every other day for a few days leaving it to soak in.
Then in about a week give the nipple a sharp wack on the top to give it a bit of a shock before trying to undo it again with a spanner.
I wouldn't risk applying heat as it could damage the rubber seal.
I'm sure somebody on here will have a better idea though
