There was one company I used for many years for shirts and shoes until their quality went downhill big time and I had to keep returning goods. It got to the stage where I didn't want to use them any more and unsubscribed from their mailing emails and and asked them to stop sending me catalogues. Within 2 weeks, I suddenly had begging letters from charities and other companies flying through the letterbox (whereas we never had any of these for the last 20 years). I was quite furious about it and did a bit of research. I rang one of the insurance companies up from the mailings to complain and stated that I had never requested anything from them. They asked for a reference number on the envelope. She looked it up and told me that my details had been bought from a certain group of companies! She removed me from all their mailing lists and apologised.
Doing some further research showed that one of the companies in this group was in fact the company I had stopped ordering from. When I rang the culprit up, they "checked their database" and denied it, until I provided the details and reference kindly supplied by the insurance company I just spoke to. They then fudged about and made some excuses and apologised. I followed this up with a threatening email under the data protection act formerly requesting action from them to remove me from all lists present and future and gave them 21 days to comply or risk being served with a DPA breach. Got a grovelling response and never had another thing from anyone - bliss!
Back to your HMRC - this is a well-known scam and similar ones are abound in the US from the IRS. They threaten you with court action or imminent 'arrest' (
How desperate things are getting!