As you have read on this forum, there are lots of people in the same boat. I too received the dreaded FRA major works bill, with little evidence of work, but a sky-high bill.
I did manage to get them to email me over a spreadsheet with a detailed breakdown of the costs, which helps to clarify what the actual scope of works was. But I can tell you that the material costs were 10x of what they are in stores.
You can email Capital Services on CapitalServices@camden.gov.uk and Assignments Team on AssignmentsTeam@camden.gov.uk to request the detailed breakdown of costs in a spreadsheet, but don’t hold your breath, it will take months.
Meanwhile, you can confirm you were notified about the intended works and the fees do not exceed the amount disclosed upfront. If they did, then you shouldn’t be held liable for the extra amount.
You always have the option of taking this to a tribunal, but I have no idea what the success rate looks like. The freeholder is meant to not impose unreasonable costs to leaseholders, so if you feel you can be bothered and have a strong case, you could try.