Extension roof maintenance costs

Hi, I live in first floor of a Victorian semi-detached house. The ground floor flat has built an extension about 30 years ago a kind of very large conservatory. They now claim the felt on roof of this extension needs replacement and they asked both first floor and second floor to share the cost.

All three flats are free-holders and have share of free-hold.

Are ground floor free-holders being reasonable asking to share the roof cost or should they not bear this whole cost themselves considering it is their extension?

Thanks,

Nic