I’m not going to recommend either way – you’ll have to make up your own mind 🙂
On a technical point, your service charge letters won’t have mentioned this, this counts as “major works” and would have been sent separately from any other paperwork.
You may well have a case, but I can’t guarantee it; in one instance I thought I hadn’t had a notice and didn’t realise that I had – I only discovered when looking through my paperwork files more closely. I’ve mentioned to Mike Edmunds who runs Leaseholder Services that the some letters are often a bit torturous to read but he says he is legally bound in most instances; the Leaseholders’ Forum are nudging him to find workarounds!
More specifically, you won’t have received things about “on going work” or a date for when you would be billed, just some form of very official notice saying that the work and/or finances for the work had not been completed yet, at least once every 18 months since the work started.
Perhaps phone up Leaseholder Services and discuss the situation with them, referencing Section 20B? There is also the complaints process should you encounter any difficulties. We’ve produced a guide about the complaints process here https://www.leaseholdersforum.org.uk/guides/ and there is plenty of stuff about Section 20B on the internet.