As you say it complies with the current guidance within the Approved Document in support of Building Regs 2010.
The Fire Authority should also have been afforded the opportunity to consult on any "additional requirements" they might have had under the RR(FS)O 2005 through the Building Reg/Fire Safety statutory consultation process; I trust they made no comment about the lack of cold smoke seals at that time and a Completion Certificate has subsequently been issued.
Given that the premises complies with the functional requirements of current Building Regulations, benchmark standards (Purpose Built Flats guidance p.69) indicate that fire protection is "adequate".
I would therefore ask if it is reasonable for a fire authority to require the retro-fitting of cold smoke seals in this instance?
Yes, technically you are correct. However, I have learnt through my experience, argue about the matters that are worth arguing about.
For the sake of a simple replacement of seals? Is the hassle worth it?
Possibly a different argument if we were talking about the replacement of doors, or maybe even routing the door or frame.
Before answering this question it might also be worth referencing the Article 36 Determination published in May 2012, where the lack of seals did not meet current standards.
Yes, it sets the benchmark, which to be fair, I agree with. Personally, I am not a huge fan of strips and seals anyway, I think they are very overrated. However, there is a totally different set of circumstances here, which would bring the validity of the determination into question.
Being a riser, are we talking double doors or single doors?