After around 2 months of deciding if I should pay out for this bag, I finally got one and haven't regreted it.
The build of this bag is unbelievable. The reliable zips around the opening is thick and durable and meet in the middle with holes for a small lock.
Inside, there is a small mesh pocket on the 'lid' with a small hook for what I pressume is for keys. Great easy access.
For the extra £10, I chose the Large, as once filled, the compression straps all around compress it to the size of a medium rather easily.
As for the material, I'm led to believe that it is made from PVC which looks and feels very thick while still retaining it's light weight.
Overall, a VERY practical bag which will be around for years to come. Would recommend to anyone.