Ok 1st and easiest way is to extend the spanner you need to tighten the flange nut if i remember those spanners are flat, get another piece of flat metal and cut it to the size you will need ie about 2in from the top of tank get it welded on to give you an extention that way you can tighten the flange nut from the top of your tank! or drill 2 holes in the top of the spanner and 2 on the extention piece and join together (follow)
The 2nd way is to cut your 65mm hole at the bottom of the tank then cut another piece of plastic into a square about 100mm then cut your 65mm hole into this drill small holes around the edge of the square apply silicon and pot rivet onto tank! the silicon will seal around the outside edge of the square piece of plastic so will not get leaks.
The first method is better less hassle!
One of these spanners davids all you are doing is extending the handle to the size that you want it!
http://www.didcotplant.co.uk/catalogue/detail.php?id=377