This is a ‘Unique Tax Reference’ which is required in all cases in order to submit a tax return. This is required as an entry on our ‘Essentials’ form at the end of the process and therefore all other stages, including tax calculation and payment summary can proceed without this number. If you don’t have one, we can advise you on how to apply and provide the applicable form for you. We do this entirely free of charge. Please note that obtaining this number can take up to six weeks and therefore this should be borne in mind with a view to the 31st January deadline to submit the tax return without £100 penalty.