How do I know what type of fire door I need as I think I only need an fd30 but was told I may need an fd30s how do I know what is the correct door to fit and one that is not over specified, the door I have is old but I believe it is a fire door as the glass inside the door has wire squares the door thickness is 44mm it has 2 hinges that are self closing it’s a solid core 

If the door is an entrance door to your flat and opens onto a common area inside the building, its most likely that the door you need will be rated FD30(s). 

Further to Neils reply, if the door in question is protecting a common area from a fire in your flat it needs to protect from heat and smoke so needs to be a FD30s fire door. It is difficult to say if your original door meets the required standard you could have it checked out by a FDIS inspector.