I've lived in this home for 20 years and never had a problem with my boiler furnace and now they tell me I require a low water cut-off valve, service people have cleaned and inspected the unit with no mention of this before, or they will cut off the gas supply if it's not repaired within 45 days ? What is the cost and is it a new requirement ?

Who is telling you this? Usually low water cuttoffs are only required in commercial buildings. However, it is always a good idea to have one on any hot water boiler system, and codes do vary from area to area, so maybe it is required in residential homes in your area? I would check into this with the local authority. It would be near impossible to give a price to have one installed without knowing what the setup of your boiler is and many other aspects of the job. Good luck and please do let us know how things turn out.