Unless you achieve a legal or contractual right or consent to carry out works to the landlords premise, the roof work is trespass and you could be liable for considerable damages. In simple terms it is therefore not your roof to replace.
Doing this as a group can have problems with liability for defects for the freeholder and you as residents and a future flat buyer. The terms of warranties are not as comprehensive as sold and are often limited to the original parties and not a new flat owner, and in the case of negligence you could be sued several years after having sold up and left. If the roof collapses the four of you are directly liable not the contractor, you then have to sue them.
So it is for the freeholder to do under the lease and they can sect their contractor as long as there are at least two estimate for the work as part of the statutory consultation'
At present neither you nor the freeholder can proceed, and your agent should know better as this explains. http://www.lease-advice.org/document..._Landlords.pdf
Your main options are
1: They end to do the consultation, then have it requoted, and if they insist on using their chap, then take it to the LVT to determine independently.
2: Or as four flats exercise the no fualt right to manage and arrange it,and everything else yourselves. http://www.lease-advice.org/publicat...nt.asp?item=21
Based on the information posted, I offer my thoughts.Any action you then take is your liability. While commending individual effort, there is no substitute for a thorough review of documents and facts by paid for professional advisers.