Its 20 million UK mobile users should check their statements, as they may be entitled to a refund

EVERY one of Vodafone’s 20 million UK mobile customers needs to spend five minutes reading this. There’s a chance you have overpaid by hundreds of pounds, and at the extreme may even have had your credit score hurt. As this email I got from Anna shows, it could be lucrative… "Thank you for encouraging Vodafone customers to check their bills. I noticed I was overcharged £330. After contacting it, I'm going to be credited with £575 as it found I was overcharged more than once."

What’s the problem?

Last year Vodafone moved to a new billing platform, and thousands have reported problems, including direct debits incorrectly set up, people being put on the wrong tariff and credit agencies wrongly being told that customers have missed payments. After investigating, my team found this was mirrored by stats from official channels too: - Ofcom data shows it’s had more than three times as many complaints about Vodafone as any other network and pay-monthly complaints about it doubled over 2015.

- When I launched this campaign in June the Communications Ombudsman warned it had seen an increasing number of complaints over the last nine months. And, indeed, Vodafone itself admits problems, saying: “We would like to apologise to any customers who have been affected by our recent customer service issues … many of the recent issues relate to the move of our legacy billing and services platforms into one state-of-the-art system”. Yes, you did read that right, it is saying, ahem, ‘state-of-the-art system’.

This isn’t the first time I’ve come across near systemic billing errors. Most often in the past they’ve been by energy firms such as Npower, Scottish Power and Co-op. And, when it does happen, the most important thing to do is to check your bills and bank statements.

How to check if you’re affected

* Check your Vodafone bill ASAP The common problems are: a) Being put on a different tariff to the one you asked for and b) Being charged double or triple what you should have been. Like Anna who Facebooked me: “I was overcharged every month for four months and had to call four times each time to get it sorted. Will be leaving them when I can!”

* Even if your bill is correct, check your bank statements too. The common problems here are: a) Direct debits being incorrectly set up, with customers then chased for late payment and b) Payments being taken after you’ve cancelled. Mary emailed me saying: “I cancelled my Vodafone contract in 2015, but they’ve now sent debt collectors saying I owe money as I didn’t cancel properly. Please advise, I am a pensioner. I know I cancelled it.” Plus if you’ve cancelled your account with Vodafone, check you’re not still paying, even if you received confirmation of the cancellation.

* Found a problem? Check your credit score too. Unfortunately, on the back of problems some people have found credit reference agencies have been told they’ve missed payments. This can have a massive impact if you’re applying for a mortgage or credit card in future. So if one of the errors above have happened to you, it’s worth checking your credit reference files. For how, see www.mse.me/creditscore.

What to do if there’s a problem

1) Work out what redress you want. This isn’t about compensation. The most important thing is to be put back into the position that you would have been in had the issues not happened. That should mean a refund of any overcharges and any expenses directly incurred as a result, eg, bank charges resulting from a fault.

If your credit score’s been affected, the most important thing is to ask Vodafone to delete its entry on your credit file. If it won’t, you can go to the Financial Ombudsman. You can also add a ‘notice of correction’, so lenders can see what the issue was, explaining that it wasn’t your fault.

But whatever you do, don’t be tempted to cancel your direct debit, as you could end up getting valid marks on your credit file for payments you do owe Vodafone – and those can’t be amended.

2) Contact Vodafone immediately. Once you know what you want, hopefully the issue can be sorted quickly. You can call it for free from your Vodafone mobile on 191, or call 0333 304 0191. Or use its online chat (www.vodafone.co.uk/vodafone-uk/forms/complaints). That way, you can save a transcript of what’s been sent for evidence should you need it. Alternatively, fill out its web form at www.vodafone.co.uk/contact-form/index.

3) If that doesn’t work, there’s a free online tools to lodge a formal complaint. To make a formal complaint, go via www.mse.me/vodafonewarning, where working with complaint system Resolver there’s a special tool to automate the complaint. As TateyW tweeted me: “@martinslewis After several phone calls and webchats with no success, got £80 back from Vodafone after using Resolver.”

4 If that still doesn’t work, or it’s been more than eight weeks, escalate your complaint to the Communications Ombudsman. It’s an independent and free scheme that covers most telecoms firms, and around 60% of complaints about Vodafone escalated to it in the nine months up to June were upheld (the automated system above does this for you). If the Ombudsman upholds your complaint, Vodafone will have 28 days to comply with its decision. If it rejects you, your last option is to go to court, though remember the Ombudsman probably sided with Vodafone for good reason.