Thanks for the comments especially DM for the link, that was great help.
Well, I got home early last night, cleared the kitchen table, covered it in a towel and away I went.
First I measured the gap between the low E and string and the first fret and I measured it as 0.6 to 0.7mm (sorry you guys over the pond who work in inches). I then placed a capo on the first fret and measured the gap between the low E and 2nd fret and although there was a gap it was not measureable with my steel ruler but I would guess 0.25mm.
So, before slackening the strings I scored around both edges of the nut with a modelling knife to stop the liqueur chipping when it was removed. Then I loosened the strings enough so that I could lift them out of the grooves in the nut and pull them to the side. Then I scored along either side of the nut with a knife.
Then with the end of chop stick and wooden spoon (improvisation or what) I gently tapped the nut from either end to try and loosen it but it wouldn’t move. Then I got my modelling knife and gently squeezed it between the nut and neck and up she pops without any force at all.
Then marked a pencil line around the base of the nut about 0.25mm from the base, I didn’t want to sand to this line straight off, I just wanted to use it as a guide to see how much I was taking off and make sure what I was taking off was even. I got a sheet of medium grade sandpaper and held it flat on the table and with the other hand rubbed the nut on it making sure there was even pressure on the length of the nut. It’s then a case of replacing the nut and tuning in both E strings and measuring the gap, it took 2 goes to get it about right so I then tightened all the strings, tuned up and had a play and it was incredible the difference it makes.
So finally I removed the nut again and dabbed a little glue on the underside and fitted again making sure that the nut was firmly set into the grove before tuning. After tuning I lifted all the strings from the guitar individually and retuned as they’d all went a little flat as I guess the nut was being forced back into place.
I can’t tell you how happy I am with the result, it’s a massive improvement even when playing open chords. The guitar seems to have more sustain and tone but that may be because it’s far easier to finger chords. One thing I didn’t expect to find was that it’s easier to barre with a capo, I guess it’s because the strings are under less stress because they are now closer to the frets?