I don't know how this will resolve itself but at least contact your bank and ask for a copy of the cancelled cheque to prove that the money was paid to him. Although banks don't return cancelled cheques to the writer any longer they do keep them in the archives for up to seven years and if you do decide to do battle on this, you will at least then have proof that the money was paid to him.
All I can suggest is that in future you become at lot less naive about your cohabiting arrangements and make sure that after "once bitten, twice shy" you get any financial loans or agreements properly drawn up in writing.. This isn't mercenry. It's plain common sense and the minimum that would be done in any commercial contract.
In your case, if your ex boyfriend paid all the bills and for your holidays you will probably find that if you lived with him for any length of time, he probably paid out a lot more than �5,000 so it's a case of "swings and roundabouts" and you probably don't have any claim on him.