Don't mean to answer my own question because I am no way certain of the physics involved however...
If rate of climb (or descent) is determined by power produced by engines, and the power is proportional to thrust (and therefore fuel flow) for a given airspeed, then it seems to me that at 210 knots, a fuel flow of 350 kg/h does indeed equate to 500 fpm.
In my example I increased the fuel flow to 700 kg/h per engine and the rate of descent reduced to 500 fpm. If I were to add another 350 kg/h to make the total fuel flow 1050 kg/h per engine the rate of descent should reduce by another 500 fpm to, well, zero.
Level flight at 210 knots with fuel flow at 1050 kg/h per engine sounds just about right to me.
BOAC I'm suggesting the performance reduces by 50%. You say Boeing says it reduces by 100%. Interesting. And yes I mean no, there is no such information in our training manuals.