The time between the erroneous payment and the request to give it back was a month. Either the bank statement would show that it was spent in that month or it will not. If the money was still in the account at the time of the request to give it back, I think the OP would have to show that he had committed it to something that he could not get out of.
Besides which the OP will undoubtedly have to fill in a form showing income, expenditure etc. in order to ascertain what can and cannot be afforded. Never lose sight of the fact that although the ex-employer may have made a mistake, it is the OP who is in default as soon as he was told of the mistake. If you gave a shopkeeper a £20 note instead of a £5 note in payment of goods, and informed the shopkeeper as soon as you discovered your mistake (and the shopkeeper acknowleged that you had overpaid), what do you believe would be a reasonable repayment period for the shopkeeper to reimburse your £15? Would you be prepared to accept 6p per week for the next 5 years, for example?
The warning *is* the start of legal action (pre-action protocol). It would probably come in the form of a solicitor's letter. The OP would at that time probably be asked to pay back the amount *plus* the cost of the solicitor. From then on, every communication will bump up the legal costs. If he remains stubborn, the legal costs will quickly dwarf the amount of the overpayment.