In fact the platform staff member was wrong. If no reservation labels are provided on a train then it's officially deemed that the reservations have not been provided. You're perfectly within your rights to occupy any vacant seat and ask anyone who waves a reservation coupon at you to go forth and multiply as politely as you see fit.
Platform assistant, as byelaws state that you must follow instructions of any employee of the railway,
Not strictly correct. There are various references in the byelaws to obeying the "reasonable instructions" of an "authorized person" but there is no catch-all byelaw that says "you must obey all instructions from rail staff". The closest is 12(2), but that refers very specifically to safety issues which this clearly isn't:
12. Safety instructions
(2) An authorised person may, in an emergency or in other circumstances in which he believes he should act in the interests of safety, issue instructions to any person on the railway. No person shall, without good cause, disobey such instructions.
Even in this case you can still argue "good cause" for disobeying the instructions if the authorized person is talking ballcocks.
The byelaw dealing with reservations is here, with my emphasis in bold:
19. Classes of accommodation, reserved seats and sleeping berths
Except with permission from an authorised person, no person shall remain in any seat, berth or any part of a train where a notice indicates that it is reserved for a specified ticket holder or holders of tickets of a specific class, except the holder of a valid ticket entitling him to be in that particular place.
No notice indicating the reservation, no reservation. It's that simple.