Does anyone know if updating the Doorset of an old fire door is classed as a 'material alteration' under part B of the building regs? Example of this might be adding a door closer and intumescent strips to the door frame. 

Building Regulations apply to new buildings or where there is a proposal to change the use of an existing building (say for example: from an office block to a block of flats). From what you say, it sounds like what you are doing is trying to upgrade the fire performance of a door.

 In relation to B1 a Material Alteration is when the means of escape (MoE) is less satisfactory than before the work is carried out. In your case the MoE is likely to be improved therefore it is not a Material Alteration. 

Check out Approved Document B (fire Safety) volume 2: Premises other than Dwelling Houses Page 11 item 0.20. It is the same for a Dwelling House.