Yeah - it took a LOT of effort to get rid of, and many, many
deaths! (at least on my part)
Many thanks to everyone that helped out
For those not online, I found it in an underwater cave system (had to dig down on the ocean floor to find it). Looks like the cave system had already been explored as there were torches inside already (along with a Wither, gradually mining it's way through). In our attempts to kill it, it somehow escaped and headed towards land (slowly, as we were taking it in turns to be "target practice" for the Wither, whilst we were in boats / swimming!).