I hope you have some written evidence of the loan.
If you want to protect your loan as much as you can, you need to take County Court action against the borrowers. (I assume the amount of the loan is now due to be repaid - if not, you can't take action until it is.) You should then - if the Court accepts the evidence that there was a loan - get a County Court Judgement. If the borrowers do not then comply with the terms of the judgement you can apply back to Court for a charging order on the house. Then, when/if the house is sold you can get your money back if there is sufficient equity after paying off the mortgage.
This process will cost you in Court fees and lose you your friends. You have to decide whether its worth it.