I am refitting a washhand basin and pedestal and have a couple of flexible tap
tails to fit to the monobloc tap..... ...I'm undecided whether to bring the
feeds up through the floor or through the wall the basin is secured to .I'm
trying to either hide the feeds as much as possible ( using plastic ) or I'll
use chrome pipe,in which case it doesnt matter so much .
Are there "rules " about using flexible tails as keeping them straight as much
as possible or is it OK to "bend" them ?

i`m not a plumber etc :-}

IIRC the monobloc tap on my kitchen sink has two small copper tails that
we had to use - there was no way a flexi would attach due to the
connections on the underside of the monobloc.

You may need to check on the connection arrangement to see if its
actually possible to get the flexis onto the monobloc direct :-}