Friday rode the return of my 120 mile commute, looks like mpg at high speed (70mph+) is around 71miles per uk (imperial) gallon. Which is the same as before the mods so a more sedate pace should return around the 90mpg I was seeing previously.
I have also obtained a pair of Dark Green 1958 pattern British army waxed canvas haversacks that can strap together and be used as panniers. I have kept the Givi pannier frames on the bike so I still have a hard case option, the haversacks straps loop over the seat and between the rear of the seat and grab handle, there is also a clipped strap that I have used to loop behind the Givi frames to hold the haversacks tight against the bike.
the carrying capacity is near double over the Givi panniers but are much slimmer when empty, ideal for what I need.