Thanks again, all, for very useful commentary and suggestions. My companion is under 50kg, so our combined weight + fuel for a couple of hours (and a toothbrush) should be within limits.
But basically is sounds as though the manufactures push the limits of performance and weight, and leave it up to the PF to make a decision on the 'limits'. That's certainly not an ideal situation - but I guess makes sense for the a/c manufacturers, given the more generous (realistic) MTOW limits in other territories.
I've flown in a Savannah a few times in Zim; and I had already clocked the Ikarus as 'one to watch'. I guess the next step is to get my Zim PPL back, and go from there.