The details of the complaints process are here: https://www.leaseholdersforum.org.uk/guides/complaining-effectively/
At Kiln Place estate, I’ve found that the final charges are broadly similar to the recent estimates (somewhere around the £150 mark), so I don’t know what happened in your case.
Ultimately, if you don’t think that the bill represents fair value then that is for the First-tier Tribunal (Property Chamber) to decide (they used to be known as the LVT). That said, you can certainly ask Leaseholder Services for a breakdown of the charges and it doesn’t hurt to ask.
For what it’s worth, the Major Works department is currently in the process of tendering for more suppliers (which the Leaseholders’ Forum has lobbied them for for many years; and worked with Camden Council to get them to this stage), rather than relying on their current two; hopefully this will bring down prices for new works, but sadly doesn’t affect work already underway or completed.
(Leaseholders’ Forum member)