Audible crackles should not get charged ... but they might only get investigated by a voice engineer rather than a broadband-skilled one.
I think it's a voice engineer coming, I'm not sure though (Zen did suggest we get the crackling fixed before worrying about the DSL, so I agreed to that). I'm sure they won't be able to deny a fault if they try to use the phone.
The more I look at the way your DS & US SNRM falls off over time after a resync says that line needs a Oprenreach appointment it's almost like a battery or capacitor slowly draining away it's voltage, if your up to it you could replace your Modem and the NTE5 & SSFP and see if that helps if not call your ISP.
I had a resync a short while ago (this time without using the phone) as the SNRM dropped to basically 0. Currently I have a rat tail filter (£10 in Maplin, what a rip off!) to the test socket and a new/overpriced DSL cable just to ensure none of the old stuff I had was the issue.
I see what you mean on the graph where it's falling over time - I've attached a screenshot for anyone interested.
I can perhaps change the modem to the one Zen sent originally if I can work out how to get it into a bridge type mode. I don't much fancy trying to replace the SSFP though as that'll likely end in tears!