ws2000 Stad

Hi. I made pension contributions over the last 4 years to a SIPP outside of my workplace pension scheme. I earn less than £100,000 but over £50,000, therefore I have never had to submit a self assessment. I have now been made aware that I am owed further tax relief on the higher rate tax for my pension contributions. How do I claim back the tax on my pension contributions given I have paid tax at the 40% tax rate if i do not need to do a self assessment? And how can i claim back the tax for the prior years? Thank you