I have ordered a 3 blade head for my Strimmer FS90R, from you. Can you tell me when I fit the cutting blade, do I have to remove the skirt on the deflector, or fit a new deflector?

You will need a new deflector if changing from a strimmer head to a blade, for the 3 point blade the below should be suitable: https://www.lsengineers.co.uk/catalog/product/view/id/16301