Interesting. In walls, cables have to follow certain routes, but that is because occupiers may bang nails in the walls. I don't think that that applies to floors.

However, if the cables are in proximity to warm pipes, an allowance will have to be made for the ambient temperature as well as the method of installation. Put another way, the cables may need to be thicker than they would be under say a suspended wooden floor.

It's the IET regs these days - more properly know as BS 7671.

There are a whole host of things to consider - from ambient temperature to the layout of the conduit - e.g. U shapes that pass into cooler areas maybe prone to collecting condensation. No problem running diagonally as far as I recall though. The conduit probably needs to be of sufficient depth to avoid nails & screws (e.g. from floor covering fixings) or be of earthed rigid steel.

   - Andy.