Ok, this is starting to get somewhat concerning as the engine on this mower always ran very well. I know its best to eliminate the obvious stuff so let me just get those out of the way.
- fuel, plenty in it - known good fuel used on other GXV160 engined mower within the past day
- cleaned and replaced plug, that didn't work, so replaced with completely new plug, that didn't work
- checked and there is a definite spark
- emptied carb fuel bowl, made sure all bits in place and clean, no leaks, good seal
- used SCA engine starter spray - still reacted exactly the same as without, started strongly but then after ~3sec cut out, adjusting the throttle up or down did not make any difference, no smoke at all
- removed the SP drive belt, made no difference
- removed the air filter, that made no difference
- oil level is correct, only changed a few weeks ago

Kind of out of ideas - never had any issues with starting this one, only SP issues. Pretty stumped so any ideas welcomed. :-/