Hi all,
After a constant battle I finally found the root of my problems - a mix between my router and it's 2.4Ghz wifi network. I came across it by dropping a .tar file to the Pi. My router is a TD-LINK TD-W9980 and although it's not positioned in a great spot (new build, master socket in a cupboard), it can only be 10 feet away from the pi with not much in the way. The transfer rate of this 100mb file was between 100kb/s - 200kb/s! No wonder it wasn't streaming anything! Checked and double checked my router settings, done a few of the old tricks - changed channels etc but nothing improved it. The PC near enough next to the router can max my 80mb fibre out on the same wifi network so who knows who to point the finger at. Apologies for the ramble.
After some more digging, I found out that the three big old external antennas on the W9980 are for the 5Ghz band only. The 2.4 Ghz band run off internal antannae and as expected don't perform as well. So the next step is to buy a 5Ghz compatible USB dongle for the RPI3. Again a complete minefield of information but in the end managed to find the one mentioned in the subject at a good price and confirmed by a couple of sources to work with RPi / OpenELEC no problems. Can I make the assumption that because it is compatible with OpenELEC, that it isn't going to have issues with LibreELEC? Just works out the box? The correct answer is yes!
And no, ethernet isn't an option unfortunately.