These must be kept until the end of the fifth anniversary of 31 January next following the tax year. Effectively 2 months short of 6 years.
The type of records required will depend on your personal circumstances but will generally include:
For those in business or letting property you must keep detailed records in respect of the activities of the business and rental. This includes full bookkeeping records with supporting documentation (receipts, invoices, contracts business mileage records).
Where there is insufficient support for the HMRC can refuse to give relief for expenses and can impose penalties (up to £3,000 per year).