So update on Mr Brown after a second opinion and fresh xrays. It appears it was a misdiagnosis by the vet I saw. It turns out the large mass was actually an extreme case of bloat and liquid build up. Second xray shows the mass is all but gone. He is now on correct meds for bloat.
To say I'm over the moon is an understatment. But I'm also furious and will be moving to a different vetenary practise that has been recommended to me.
Charlie is happily alseep in his hammock looking alot thinner and weighing 197g less than at the weekend.