Tamiya Thinners X20

X-20 or X-20A? There are two types, depending on where you're located and if it's imported there.  X-20A is an acrylic thinner, but X-20 is an enamel thinner.  X-20A always has a white cap, X-20 blue capped (except the 10ml size, which is clear).

X-20A is alcohol based, but X-20 is essentially mineral spirits/white spirits. Your pic looks like what I'd expect if you tried to use X-20 with Tamiya acrylics.

Its the X-20A, been asked by the UK distributer to send it too them as they think there is a contamination issue, may be from the X-20 from what your saying

If cross-contaminated, you may see a demarcation as the alcohol based X-20A and oil-based X-20 will not mix.  

Another thing to consider is the possibility of cross-contamination in your airbrush (if you use both acrylic and enamel paints)