On finding a vendor, he'd pay back the loan and the rolled-up interest and split the profit 66:33 with the building society.

Can be interest-only for a period, followed by interest plus principal, or a balloon payment (with rolled-up interest) at the end of the term.

For example, if Newco issues deep discount securities under Sch 4 TA 1988, no or little interest is paid to the lender, but when they are redeemed the discount element becomes payable (representing, effectively, rolled-up interest).