There are two areas of income that you might have received, and that therefore may need to be declared: -
1) Any interest paid to your husband by the son on the 20% loan (maybe you didn't charge him any - that's OK). If you did take interest from him, it goes in the same boxes in the tax return as Building Society / Bank Interest received.
2) Any capital gain made on the property. It isn't just sufficient to say to HMRC 'we loaned our son �20k and took a stake in the property - now he has paid the �20k back and our name has come off the title register'. HMRC will want to know that that the property has not increased in value between the time you bought it and now. The way to do that properly is to have it valued now (unless your son has just sold it? - but it sounds like he hasn't). If the property was worth �100K when your son bought it, and it is worth �110k now, then you have made a 10% capital appreciation of whatever stake you provided him at the time.
There is a capital gains annual allowance before CGT is payable, and unless the numbers are large it is unlikely that CGT is due - particularly as in many parts of the country, property prices have fallen recently. CGT is payable at 18% of the net gain, I believe.