I would hope that it would be since the error first occurred, but I think that having more specific information would help. Also it depends on how you design “significant” and whether Camden agrees.
As an example, Camden don’t always fulfil their obligations under the lease (things to do with cleaning windows, but I could easily misremembering the example). They don’t charge for this service and therefore they say that the material effect is “nil” – because you aren’t being charged, you don’t lose out.
In the first instance, I would personally email Camden, see what they say and then ask them to justify this stance. That way you have everything in writing. Ultimately the First Tier Tribunal (property chamber) would be the referee.
Here’s Wiki on the Statute of Limitations: https://en.wikipedia.org/wiki/Limitation_periods_in_the_UK (no responsibility for accuracy of lack thereof!)