Any question answered...

I reckon that weight would effect acceleration though, the heavier bike and rider would accelerate faster (this may be hopelessly wrong)

You're right, it is hopelessly wrong. From a standing start, with no resistance (all resistance to bicycle motion is zero at zero velocity), initial acceleration is the same for any mass of rider/bike.

If the hill goes on forever, you eventually reach the equilibrium velocity where the power input, which is the g times total mass times the rate of vertical descent equals the power output, which is the sum of aerodynamic drag and rolling resistance, of which the former is dominant at speeds over about 5m/s. Equilibrium velocity increases with mass but decreases with CdA. Since people all have roughly the same Cd, if they adopt the same position, and A rises as the square of height but mass rises as the cube, big blokes tend to get to higher speeds on long descents.