Thanks again Matteo,
I think I have misunderstood medieval economics.
With further reading Fourteenth Century England Vol 2 by Chris Given-Wilson, I think the English Florin failed not because it was inferior to the already circulating Italian Florins, but due to the disparity in silver & gold prices, so no one was submitting silver to the mint to be turned into gold florins. So there was no demand for them to be minted rather than there was a reluctance to accept them in trade.
From Fourteenth Century England Vol 2 " The new coins were overvalued in relation to silver, and the King's profit or seignorage of £1 per lb. wt. was too large."
Ian..