Further to last:
I did a bit of research and found this http://www.albaplc.com/PDF/IB%20PDF%20files/JCB/Line%20Trimmers/JCB-PLT25.pdf which anyone else reading this should find useful.
I figured that the only way I way going to get into it in my case was to hack the plastic line release button off the end and dig out the spool until I could get a grip of the shaft. Well I couldn't find the cutting bits for the dremel so I just used the "dentist drill' aka large engraver bit. This melted the plastic more than it drilled it. I ripped off the aforementioned parts [Look on page 17 in the MI's] Then managed to grip the shaft below the square nut that was revealed. It came off easily after all that and now I need a complete new end [as per figure 26]
Did I learn anything useful? Er yes as it happens. If you could see the back of the spool holder there is a plastic part which holds the 'captive nut head' which is actually part of the drive shaft and what the line release button is meant to turn against. A pair of mole grips here perhaps with a bit of 'plastic hacking' may help the next victim of the JCB plt25 to get a purchase on the shaft. In my case this captive part of the spool holder was rounded out and the drive shaft nut was spinning inside it.
Heres hoping my experience can save someone else the chew that I have just endured.
Slugbabydotcom
May 2008