Same problem here, DD is VERY sensitive to all this gunk & has ended up in A&E with what turned out to be Colitis as a result of it being prescribed before
...
our old Chemist was great, knew here problems & would automatically mix up additive free stuff for her, but we;ve had HELL & its been a real learning curve since he retired, as it turns out they are not allowed to do that any more, most just don't know how, & those that do need a special (expensive I'm told) licence 
I've finally after seeing gastro Dr & trying to get our GP to help, (waste of time bar passing me on) seen a hospital dietitian, she was brilliant, knew the problems these chemical additives can cause (many can disbelieving the reaction can be so severe) She tells me its a new area of research with the NHS, but 1 they are realising is important,
she is finally sorting out letters to our GP telling her exactly what to write on prescriptions so that they are send of to a central pharmacy that will mix up additive free, it comes back the next day....so far the problem has been, the Chemist needs it written in a certain way, but doesn't know what as it varies according to the drugs needed, & GP had no idea what to write....though took seeing a 3rd GP to finally own up to that
In the mean time I did thankfully find a Chemist who, though not able to mix up, as not lisenced, came from a long line of Chemists & at least could explain it all to me :)..........he said the same as jasper above, but also went on to tell me to crush & mix it with neat RIBENA....apparently it was developed by Glaxo as a mask for Kids medicines, but it was so popular they marketed it as a drink......it will hide the taste of anything :)