Avoid Tax Repayment Agents

When i was working with Students, every year i would hear of them being ripped off by Third Party Tax Agents who claim a tax refund for them but then claim as much as 40% of any repayment as well as an administration fee for doing as little as passing on your pay and tax details for the year to HMRC and asking them to pay them any refund money due.
I mentioned it to HM Revenue & Customs who said they couldn't do anything about them because the people would sign a form to say they want such a company to claim the any tax refund due on their behalf so it was legally binding.
There was even Tax Agents contacting HMRC to remove things from the tax code in the year such as Expenses or Marriage Allowance to force the client to pay too much tax in the year and generate a refund so they could then claim it on their behalf and trouser the 40% again.
I thought of this today when i heard an HMRC Advertisement on the radio stating that you do not need to use a third party and how easy it was to claim a refund for yourself and get 100% of the repayment due.
I did contact one Agent called Rift and they said that they offer a full tax service to individuals but they refused to tell me how many of their staff were taxes trained or if the company was approved by HM Revenue & Customs so i contacted the people at Tax Aid, a Tax helpline and heard some frightening stories.
They explained how they are constantly getting calls about these Repayment Agents and how they have been suckered into handing over a significant proportion of their refund, or even worse landing a tax bill.
The agents sign-up processes is such that if you email them or use their online calculator, they take that as you signing up and they send the details to HMRC and some of these agents request repayments for the past four years meaning that any refunds for those years will be paid to the repayment agent but here is the kicker.  
HMRC process the claim and make the repayment but they check it later, and if they disallow some of the claim they will write to the taxpayer and not the repayment agent and demand repayment of some or all of the refund and that includes the portion you never saw as the Agent took as payment for their 'services'.
Some Tax Agents also ask HMRC to put people into the Self Assessment system unnecessarily as it can be quicker or easier to get a refund that way, as opposed to claiming through the PAYE system which means not only are you now required to complete a Tax Return each year but the Agent can include expenses claims for significant amounts, again if HMRC later find it to be incorrect, you will be required to repay in full and any possible Penalties and interest for now sending in subsequent Tax Returns.
The TaxAid advice then is to avoid these Repayment Agents like the plague and you can go straight to HMRC to request a repayment, it's free and all you need to do is fill in some details online, send in a letter or call HMRC on 0300 200 3300.
If you see adverts from places like Rift, Taxback, Tax Rebate Services, Swift Tax Refunds, Tax Refund Company, Hidenda Tax, TaxPro or TaxScouts then unless you like handing over your money, avoid using them.