The notes are believed to say something like 'please give me a call, if you would like to discuss,' followed by a first name and telephone number.

An HMRC spokesperson said: ‘We have drawn on research demonstrating the effectiveness of concise handwritten notes, on top of a full letter, to help our customers understand the options open to them. 

‘We always act firmly but fairly with customers involved in tax avoidance and always suggest that they speak to us. But where money is owed that is needed for public services, it’s right that we chase it on behalf of the majority of taxpayers who pay their tax on time, every time.’

The spokesman said HMRC keeps ‘meticulous records’ of all such communication and reported that ‘this approach has proven very effective in terms of helping customers exit avoidance schemes’. Despite the apparent success, HMRC says it is not releasing any numbers.

However, the latest report from the government’s behavioural insights team (BIT), which was set up in 2010 to look at ways to encourage behaviour change in a number of areas, does indicate that HMRC is seeing improved compliance from other kinds of taxpayer interaction.

BIT’s Update Report 2015-16 says it has been working on new aspects of tax compliance. These include trials of techniques designed to help people avoid being late with their tax in the first place (rather than improving the pursuit of late payments); moves to influence corporations (not just individuals); and the use new communication channels (like SMS prompts).

The first trial focused on people who were due to make a self-assessment payment at the end of July 2015. The aim was to help prevent people from incurring tax debts and fines by prompting them before the payment deadline.

Ten days before the deadline, people were divided into two groups. In the ‘letter’ group, people were randomly allocated to receive a reminder letter, or no reminder. There were three different reminder letters, for those who were new to self assessment but had missed their first payment, due in January 2015; those who had been late several times in the past; and people who had been late in the past, but had made their last payment on time. Research showed that receiving a reminder letter significantly increased payment rates. It increased ‘new’ people paying by 34%; ‘recurring’ late payers by 59%; and ‘reformed’ previous late payers by 22%.

BIT say these effects endured even after HMRC had undertaken other enforcement actions: three months later, the payment rates of those receiving letters were still higher.

In the second SMS group, BIT divided individuals into the same three subgroups, but used two different messages for each. One was a standard reminder about the upcoming payment deadline and the other added in feedback on the recipient’s behaviour (i.e. ‘you were late last time’).  This was to isolate the effect of giving feedback on past behaviour from the general effect of sending a reminder.

BIT says the results show that the feedback message in the ‘recurring’ group resulted in a nearly 50% increase in payment rates, which it said was interesting because it would be reasonable to assume that this group would be the most difficult one to influence.

Similarly, BIT trialled sending two kinds of letter to businesses that had incurred a corporation tax debt for the first time.  One contained a social norm message, and included the phrase ‘The great majority of business owners who trade in your sector pay their tax on time. Most people with a debt like yours have paid it by now.’ The second letter was educational in content and provides tips to avoid errors in filing.

BIT found both of the letters significantly increased the proportion of businesses paying in full, while the educational letter was more effective than the social norm letter, which increased payment rates 15%.  Both the new letters were more effective for businesses with larger debts.

BIT Update report 2015-16 is here.