radiator pipes under concrete floor

For hot & cold pipes, you'd be wise to insert them into some pipe lagging first, as a precaution against frost damage and also to prevent expansion etc cracking the concrete and to stop the sement attacking the copper (if you decide to use copper instead of plastic)

what about running them in some 50mm polypipe?

That'd be just fine.

You can if you like, but you have to think about future access, IE removal, refitting, repairing etc should you ever need to. Getting at them under concrete is bad enough, doubly so if they are also encased inside a plastic pipe...if it were mine (and what I've done before) I would clad the pipes inside an oversized length of pipe lagging and put it all into the channel, then run a bead of expanding foam down each side to seal any gaps, then put 1 inch of concrete or strong sand/cement over the top, this way, if there's a leak, you can get to any section of it just by removing the 'cap' of cement and cutting away the foam...the foam also takes up any expansion/contraction.

If the floor has not been concreted yet, just prop the pipes up before concreting so that they are just below the surface...you don't want to start using a sledgehammer at some point in the future...

sounds good thanks

If these are plastic pipes, inside a plastic pipe, to be replaced by similar plastic pipe at some time in the future, it may well be possible to simply slide the new pipe in after pulling the old pipe out.

You're supposed to make them removable according to building regs. You can do this with plastic by running them in a sort of corrugated plastic pipe which you can get from BES and other plumbers merchants. I think it's made by John Guest aka Speedfit.