Never paid tax....

confused_I_think

Sirmyser, I was in your BIL's shoes a few years back though my "off the radar period" wasnt quite as long. I thought that HMRC might be a problem so I prepared very detailed 'books' which I then submitted to an accountant so that they could deal with HMRC. My books were accurate and cross referenced to bank statements etc and HMRC were fine, they just presented what I thought was a not unreasonable bill which I paid in full.
NI were more of a problem in that I recollect verbally agreeing terms to pay what I owed. I was actually paying off my arrears faster than agreed when, I recollect, they decided they wanted the whole lot in one lump sum, I couldn't do that and just continued with the agreed terms.
Around the same time HMRC 'caught' someone I knew who was in the same boat and, according to the gossip, their first demand would have meant him selling his house. I heard he ended up employing an accountant, presenting books and, I beleive, getting the bill reduced but he was a very nervous chap for a while.