Rescue dog! Big let down!

please help.
We recently recieved a home visit from a large labrador rescue centre. I explained to them that we wanted a dog who was good with children and dogs as we have a 6 year old lab and a 5 year old daughter. they noted that the temperment was our only requirement, we didnt mind a !!!!! or dog, etc. 2 days later, surprise they found us the "perfect" dog, rescued from Ireland and would fit in with our family fantasticly. We collected the dog (lab cross) paying a "donation" for £175 (donation however that was the price we were told to pay) & 4 days later he savagely attacked our lab (totally unprovoked!), I had to pull him off my dog who recieved 4 large wounds. Really upset and shocked I contacted the centre, lady very rudely told me to wait 2 months and to give the dog another chance. i contacted my vet who advised me to remove the dog as he clearly had severe behavior problems, possibly possesion issues and would possibly attack our daughter too. We returned the dog to the centre, all very upset and shocked, took my dog to the vets for antibiotics etc for his wounds. The centre refused to refund all or part of the £175 stating that it was non refundable. I explained that we had needed a dog and child friendly dog and it was clearly not good with dogs, hence the mis sale. They were rude and arrogant, I contacted the chair man who agreed to refund the money as it was a sad outcome. 4 weeks later, no cheque contacted by the lady who is a trustee who placed the dog with us in the begining, she was so rude and agressive, called me a bad dog owner, that she would never refund me the money, it is a charity....... blah blah blah..... she over ruled the chairpersons decision. I wrote to the charity, contacted my vet who also wrote explaining that it was correct to return the dog due to the scale of dog attack etc, and I have had no response, not even a reply.
I dont know what to do as I am furious with the attitude of the charity. i realised that i had to pay £175 for the dog but I didnt realise that the charity did not find a suitable dog. It would have been dangerous to have kept the dog. we now are £175 worse off, and with no dog. i am seriously considering going to court as i feel so strongly. They have accepted no liability with placing this dog with our family. They have not offered us any refund at all. Just because it was a non refundable donation I dont feel that it gives the charity the right to sell dogs and place them hap hazardly in homes, risking safety etc. please let me know what you think. I know a dog falls under the Sale of Goods act 1979 so therefore must be fit for purpose, but paying the "donation" does that rid all liability from the charity.
also the charity resold the dog to another home 1 week later for £175 to a home with no dogs therefore the charity have sold the same dog for £350! i thought it was a charity who's aim is to rehome unwanted labradors not to make a business.... .

any idea if i have a case to go to court as I feel like they are bullying me because they are a charity etc,