Thanks again to everyone for your input and giving me the pro's and con's of various materials.
In conversation with SWMBO who is also now a user of the workshop we have decided to go with a decent fire retardent plasterboard. This is because it's one of the cheaper option, will be the easiest to get a nice white finish, which is important as we have decided against windows (other than whats in the doors) and we like the safety feature of the fire retardant qualities, which in our case is relatively important as we have timber fences and a number of large trees relatively close on the neighbours side. In event of a fire, it would be nice to know it would be contained to a degree for a bit longer.
I've started the insulation process today, whilest looking at wickes for the plasterboard, I saw there slab insulation and it came in quite a lot cheaper than the equivelent stuff from 'seconds and co' so I managed to go down today and pick up a few packs and have started the installation, I'm only half-way done and it already feels better, it's a lot less echoy as well which is nice.
So, again, thanks for all the info! Much appreciated.