Hi Miccael. You need to speak to your solicitor as they should have warned you about forthcoming major works when you bought the property (depending on how long you have been living in the flat). You can speak to Leaseholders Services within Camden Council to find out when they first issued the notice that there would be forthcoming Major Works as they would have had to do statutory consultation.
Regarding the charges, it’s during the consultation process that you need to start fighting this, so it may be too late, unless the works are substandard or there have been significant costs over and above the additional estimate. Here’s a document we’ve written, but is due to be substantially revised later on this year, on this matter: https://www.leaseholdersforum.org.uk/guides/heating-options (it particularly focuses on heating systems, but the principles are the same)
Others may have more advice!
p.s. If you have problems paying online in installments call Leaseholders Services and they will be able to guide you.
Best regards, Paul