If the situation is as you set out, the advice you were given by the repair line is wrong. It is for Camden to repair the frame, and it is possible that there was a warranty on the frame which still applies and would cover the cost. Even if not, you are liable for those costs set out in your lease, which will be part only. I suggest you complain online forthwith about the way you were treated and the advice you were given. Go to https://www.camden.gov.uk/ccm/navigation/council-and-democracy/having-your-say/complaints-and-enquiries/ and take it from there; you will get an e-mail setting out the wording you submit. If you can set out the date the new window went in, so much the better.
If there is any delay in dealing with the complaint, see a Ward Councillor and ask them to raise a Member’s complaint about that.