Apologies for the newb question, but is it possible to edit an .esp file (created by someone else) to change armour in that .esp to use an accessory slot instead of the standard armour slot (and vice versa) using the GECK?

If so, are there any guides or tutorials for this for someone who has never used the GECK before?

Yes. Pretty easily actually.

You double-click the armor in the object window, and then Highlight the 'Biped Object' you want it to use. One click for 'on' and one click for 'off'.

You could check the video tutorials pinned to the top of the forum on how to use the GECK.

Thanks for the info. I will have a try and come back here and post to let you know when I have broken my game, mods and possibly the GECK how I have gone.