Norah you either pay the interest monthly as you would with an interest only mortgage or the interest gets added to the capital you borrow instead, so that you make no payment and you then pay interest on the new higher balance.
An example
You borrow £10,000. First year interest is £1000, which you roll up, so at the end of that year you have borrowed (£10,000 + £1000) £11,000. The next year you incur another £1000 interest on the £10,000, plus interest on the £1000 interest you didn't pay the previous year, which is £100. So the capital you pay interest on the second year is (£11,000 + £1,100) £12,100. so each year the balance owed by your estate rises by the added interest, on the new cumulative sum. So the debt owed back increases with each year.
As far as the value of her assets go. She is still living in the same house with the same high value but when she dies the money she has borrowed - and spent - will need to be repaid so it is a debt to be paid from her total estate.
Let us say her house and any savings are worth £400,000 and she borrowed £75,000. The value of her estate after repaying the equity release loan, for that is essentially what it is, will reduce the value of her estate to £325,000. Inheritance is paid on the value of your estate after paying all your debts. so no inheritance tax will be due.
A pension is an income and dies with the death of the recipient, so that does not come into any calculations on inheritance tax.