Please help = No backpay AND less money

Very angry right now

ESA penalised me back in October 2016 for missing a single telephone appt. Since then I have only been getting about £24/week vs the £100/week I was on previously. I phoned my useless Seetec advisor and asked why the sanction had lasted so long and he said it would be rectified and that I would receive a backpayment.

However, today (about 3 weeks since I phoned) I received a letter telling me:
"We have looked at your claim again following a recent change. From 26 October 2016 your ESA will be £29.05 a week" It also says this applies only until the 11th of April.

Is this just an underhand way of saying "we're not giving you the backpayment you are entitled to"? I'm pretty sure sanctions are only allowed to continue for a couple of months at most, maybe this is why they have worded it like this.

They can't do this, surely? What can I do about this? It's stressing me out beyond belief.

I have not missed a single tel appt since the one I mentioned. I do everything asked of me. I suppose they had cause to sanction me in the first place (because I missed the original appt), but not for more than 5 months! I still think they are just trying to weasel out of the backpayment owed to me.

They really don't care, do they? I feel suicidal. I give back to society by volunteering, what exactly do these people contribute?

I'm contacting the DWP tomorrow. I don't see the point challenging the decision to sanction me as that was legitimate, but can I challenge the amount of time I have been sanctioned?
Thanks for the help