Would I be able to place some sort of efficient, portable, crystal-type dehumidifier in my loft - one that doesn't need emptying - to get rid of slightly damp, musty smell which I could re-charge when necy (I do, from time to time, leave the loft hatch open but this can cause problems with the family!) If so, is there a particular one which I could research? Thank you.
Sounds like you need to vent your roof to eliminate this problem.
I would suggest installing breathable felt sections as this has worked on numerous properties for me and is a simple process that does not leave holes anywhere in the roof.