Is it legisltaion to have factory over head gantry cranes loewered to the ground and serviced every ten years .

Again...No

Providing the Crane is serviced ,maintained and thoroughly examined by a competent engineer at the correct intervals, why should you try to lower it to the ground every 10 years.

Still a No

I agree with all of the above... I assume that it was commissioned into use and has a safe working load clearly marked?

That seems a little odd. The deflection of a beam for a given load depends on its geometry and Young's modulus. The Young's modulus of the beam will not degrade with time, so the deflection under load should remain the same indefinitely.

Or is the competent person asking for this as there is insufficient evidence of the correct identification of the SWL in the first place?

Unless there's been significant corrosion of the members, in which case both stiffness (ie deflection) and strength would be affected. But if it is that badly corroded, I'd expect alarm bells to be ringing about the standard of maintenance anyway.

Even if it were necessary to do a deflection test, I'm not sure why you wouldn't do it in-situ: you'd either use an optical level or a laser beam. Doing it with the beam in-situ requires work at height, but in this case the hazards associated with that (if planned properly) are not necessarily greater than the hazards with lowering the crane beam and then raising it again. I'd certainly consider doing it in place.