Hi Baz,
You'll find a mass of different opinions regarding tuning, but I would suggest starting with C6 on an 8-string lap steel, tuned as per Tony's advice somewhere in this thread:
https://www.thebritishsteeliessociety.co.uk/viewtopic.php?f=34&t=9111&p=44011&hilit=+tuning#wrap.
C E G A C E (from lowest to highest) is the 'standard' 6-string lap steel C6 tuning, which will allow you a lot of flexibility, but for 8-string I would go with Tony in using C E G A C E G D (with the top D pitched between strings 3 and 4), or if you're after a more 'bluesy' sound maybe consider Bb C E G A C E G which gives you a nice 7th chord on the lowest 4 strings.
Ultimately there is no right or wrong. Which upsets my Engineer's brain, but it's something I just have to accept!
Hope that's a help,
Sid