Got everything working perfeclty now. I note - that although it does not say, if I make a (major'ish) change in the tvheadend server, a reboot and refresh in the Kodi PVR database is required, if not I get some silly errors (like no recording in Kodi). Also, I noted that I had to change the permissions (in the Java filemanager in Webmin) to the new folder I created on the new formated partition (again, followed your instructions for this) in order that Kodi/tvheadend would work (well, record). The folder had root permission (and owner), which I changed to hts... Rebooted the media server and all good...
Note I tested the tvheadend streaming (via remote desktop) to my house in Grenada W.I. (bought with lots of hard work over 30 years I add) and its brilliant, no buffering, just perfect land and sat TV.
Also, just in case your are interested, the mini HP server was brought brand new for £120.00 on the internet, they are so cheap!!! (and the HP quality is amazing as ever)... All I did was add a couple of decent part SDD/part spin hard drives... (£79.00 each - 2TB - Amazon).
So, what next challange are going to throw at me then ;-)...
Have alovely bank holiday W/E.