If HMRC really get into you it can be a big problem.
Not least because they can make an assessment of the size of your cash business - often without hard evidence - and it's then up to you to disprove it.
Disproving transactions you say you haven't made is all but impossible.
Some farmers got into that difficulty over farm gate sales.
HMRC have a "little black book" of expected takings/turnover for all types of business in various locations and cross check submitted accounts against expectation - too far off the mark and you can expect them to take a longer harder look at your business.