hello I am a lawyer with over 15 years experience. I will try to help you with this. A contract does not need to be in writing to be enforceable in law. There will be issues surrounding proving the existence of the contract without a written agreement. However it is not impossible to do so. You have the solid fact rhat you contributed £90.000 to purchase what I take to be the business premises and yu have a witness to the formation of the contract.
Are there no documents at all about this agreement, no letters or emails where this loan is discussed? Any letters or emails to your nephew's parents or other family members? Also why does your nephew say they got rid of the director? This would be relevant to his credibility as a witness? What did your witness actually see and hear?
Ok thankyou for that. I think yo have a decent chance of claiming the payment of the money yuou lent plus most/all of the interest promsed.
You might think the right thing to do is to contact your witness the former director and see what he remembers about the arrangement and to check his willingness to help. He is a key witness obviously. Your next step will be to send a letter to your nephew reminding him of the arrangement and politely requesting payment of all the money owed. If there i sno saroisfactory reponse you shoudlsend a letter referrign to the ealrier one repeating your cliam and theratenogn legal actrion if he does not respond wihtihn a set priod of time (say 21 days). If there is no response then you should start proceedings to recover the debt. If you follow this link and go to Annex A http://www.justice.gov.uk/courts/procedure-rules/civil/rules/pd_pre-action_conduct#A you will see guidance on drafting such letters.
Although you are able to bring an actin your self and send letters yourself it would be a very good idea in this case, bearing in mind the amount of money at stake and the apparent trickiness/dishonesty of your nephew to instruct a solicitor in the orthodox face to face way to help you with this. This way you would avoid the risk of making technical errors that might cost you in the long run. Follow this link to find an appropriate lawyer local to you. http://solicitors.lawsociety.org.uk/
The strength of your argument is that there will be irrefutable evidence that you provided funds to assist in the business. Your assertion that you would receive consideration for this is a reasonable one. You will also hopefully have the help of a witness.
I hope this answer is helpful but if you need any more information then please let me know.