My guess is that its a combination of the wheel, inner tube and the tyre, however ....
I finally got myself a folder (cheap one at Halfords, their own brand, "Apollo"), its okay, just the one gear, but it folds small enough to fit on the train easy enough.
The tyres were marked up as maximum pressure of 2.8 bar (whats that, about 30 psi?), which was turning out to be terrible - it couldn't take corners at any speed, bumps were a nitemare.
I pumped the tyres up to about 4 bar, and this morning it was fantastic - massively less effort required to get up to the (low) maximum speed of the bike.
So, back to the initial question - is the indicator on the tyre a reliable measure of the maximum pressure?