Overcharged on new contract, impossible to contact customer service

HI, I'm at my wits' end here. Been with VM ever since they took over NTL, didn't think customer service could get any worse, but now it's just impossible.

I won't go into all the details of dud links, phone cut-offs etc – just the main point that I agreed a package and price for a new contract, received documents both for that price and for a higher one, got assurances on chat that I had the lower price – then got a bill showing a higher charge and also an extra charge I hadn't been informed about.

Please, any help sorting this out would be greatly appreciated. Thank you.