Freehold "landlord" refusing permission for renovation work - unreasonable?

Hi there,
Would really appreciate some advice...
I own one of 6 flats that were converted from a large detached house. All 6 of us freeholders comprise the managing company/landlord.
The flat is a very old Victorian one and hasn't been touched for decades. I want to install gas central heating but because the mains gas meter is at ground level, and my flat is on the first floor, the existing pipe work isn't sufficient for my new combi boiler. The gas man has been unable to follow the track of the existing gas pipes (they go underground and disappear into the house somewhere!) and has suggested that I run the new pipes that are needed up the wall at the front of the house and into my flat at floor level.
When I put this proposal to the other freeholders one of them objected saying it would look too messy.
I'm now in a situation whereby I can't start the work without a licence from the Landlord. Is the Landlord being unreasonable in withholding their consent? What are my options?

Thanks in advance for any advice/insights you can offer me.