On the information you supply, it appears that the management company believe the roof space to be a common area to which the RR(FS)O applies.
I would suggest that if the fire barriers between each dwelling are in good condition and if they do not contain doors that enable people to move from one dwelling's roof space to another's and if the barriers are in line with the walls that separate the dwellings then the roof space is not a common area and each dwelling has its own individual roof space to which the RR(FS)O does not apply. Therefore no fire risk assessment is required.
On the other hand, I understand the management company wishing to check these fire barriers to ensure that their asset is protected adequately. This does not necessarily mean that the homeowners have to pay the bill but this may be determined by the terms of the lease.
The homeowners will benefit from this check, of course. Their property will be protected.
One of my 'ifs' above was 'if the barriers are in good condition' and we have to ask, how do we know if they are in a condition that will ensure they will perform as intended unless they are checked from time to time? This means that the applicability of the RR(FS)O cannot be ruled out.
On the whole, I'd say that, unless the homeowners are all absolutely certain that these fire barriers are still fit for purpose and that they do form effective fire barriers between dwellings, it would be a good idea to check them whether the issue falls under the RR(FS)O or not.
As for paying, check the terms of the lease. Also, don't be ripped off by the assessor. This is not a complex job and should be quite quick to undertake. If you are not happy with the assessor that the management company is proposing ensure that other quotes are obtained.
Stu