Tax and self assessment on a family loan being repaid

I am not currently registered for self assessment as my only income is from a pension that is below the personal allowance, and savings interest that is below the savings allowance. Many years ago I gave a family member a loan to buy a house. It was registered as a charge on the deeds, with the principle sum equal to a percentage of the eventual disposal price, but no other interest charges. Their house is now being sold and I will receive repayment of the loan before the end of the current tax year. The difference between the principle sum loaned and the sum repaid is about £90k. I am told that this will be taxed as income rather than CGT. If so, this will push me from a zero rate tax to a higher rate tax band for this year only. 1. I assume I will have to register for self assessment? What is the time limit for me to register, and by when will I have to pay the tax owed? 2. I currently pay £3600 into a SIPP. If the loan repayment is treated as income, can I pay a much larger SIPP contribution this year and claim the higher rate tax rebate through the self assessment?

Hi

Tax relief on payments into your SIPP are limited to your employment/ self employment/partnership income, up to the sum of those parts or the £60000 threshold (£40000 for 22/23 and earlier) which ever is the lower.  
The difference between the initial loan sum and the sum repaid will be taxed as interest.  If the interest is greater than £10000 when it arises, then is should be reported in a self assessment tax return.

Thank you