so you're putting up the money for this person's property investment? a bank doing that would insist on a legal charge over the property in the event that the person defaults on repayment (ie, a mortgage). you'd be crazy not to look for the same thing. a promissory note without any way to enforce it to get your hands on the asset is useless. of course you've only given very brief details and there could be all kinds of issues. my advice to you would be very careful especially if family/friends involved. its all fine until something goes wrong...
as for tax, it should be possible to structure this so that your profit is taxed at 20% but NOT if you structure it as interest on a loan. also technically the person who pays you the interest has a withholding tax obligation.
get professional advice