Which components actually make a difference?

So, in summary. Although it's possible that the design and quality of many bike parts will make a measurable difference to the efficiency of the cycle when ridden, most cyclists do not possess effective enough senses or athletic ability to notice.

Given the farrago of circus like activity that constitutes the majority of London's bicycle commute, this is an unsurprising conclusion.

The depressing fact is that half decent budget parts will perform better, when installed with care, than even the most expensive top end stuff, if put together by a clumbsy mechanic.