Light(ish)weight Pompetamine

Not sure how light the Genesis bikes are - I'm guessing not terribly. Couple of friends ride the Flyer as their main road bike, but they're fitter than me.

I'm under 11 stone (10.5 at fighting weight) but sadly lacking in grunt, which means those extra kgs mean a lot to me.

...so you really think an unfit person on an 8kg bike is going to be considerably faster than an unfit person on a 10kg bike? Just ride and develop grunt.
If it were me I'd just buy a complete Pompino and make it faster by swapping out the bars for some road drops, sell the wheels and buy some lighter ones and most importantly: tyres with low rolling resistance.
You *will *be faster on a 10kg bike with something like Michelin Pro 4 than on an 8kg bike with entry level Schwalbe Lugano or Vittoria Zaffiro.