Owners' manual states 87 octane for the '09 twins, which is "regular" around here. I use that in my Bonneville too, which the manual says to use 89 octane. I found absolutley no difference in milage or performance - I only have more money now. 
Higher octane doesn't mean better quality (or more power). Higher octane means the fuel is harder to detonate. Higher compression engines need fuel that is hard to detonate. The reason higher octane fuel is more expensive is not because it is higher quality but because it is harder to make. Generally the 'power' you get out of the same quantity of fuel is the same for all grades and if an engine is designed to run on a particular grade of fuel, using a higher grade won't usually have any effect (other than on your wallet!).the dealer said to use premium (95 octane) although we have (98 octane) fuel available in OZ, ..it seems like a better idea to use better quality fuel...any thoughts please?
curious...the bonneville/thrux owners manual says use fuel "with an
octane rating of 91 RON minimum"
http://www.triumph.co.uk/media/900_OHB_UK.pdf
curious...
my bike (2006 Thruxton) didn't come with a manual as I bought it at auction.
I have always been using the owner manual I found at http://www.triumph.co.uk/media/900.pdf
which says 95 RON (page 31)
but on your link it says (on page 32) 91 RON
assuming your link is for fuel injected models(?)