You can just use a local welder or a specialist in security grilles but in our experience it is far better to work with a specialist because:
Fire-escape specialists will be able to handle all aspects for the job from drawing up the plan of what needs doing, to fabrication of new parts off-site to welding on-site and finishing with the most effective protective paints
Fire-escape specialists will know what the fire-officer standards on, for example, strength of material to use, fixings to use, width of treads etc.
Visit the Fire Escape Ltd website for more information on Why should I employ a specialist?