Bellsie wrote

"I have a Trinity College octave mandolin, tuned GDAE, i.e. an octave lower than the mandolin. I wonder if i could restring it as a mandola (CGDA) and if so, what guage of strings to use? "

If you know, or can measure, the gauges of your strings you can use this calculator

http://www.mcdonaldstrings.com/stringxxiii.html (scroll down for the good bit)

to find the string tension on your instrument. It should then be possible to calculate what size of strings will have the same tension at the new pitch.

The calculator is set up for guitar but can be used for four single strings which you would double for unison pairs. Octave courses would be more complicated.