You could send them (your LM / HR / payroll) a formal notice that they owe £xx for dates yyy, based on overtime statements already submitted (attach copies so they have everything in one correspondence).
State that you have submitted these amounts through the appropriate company process and therefore you need them to confirm back by xxx date (whatever you feel is reasonable based on when you send this to them) when they will be able to settle the amounts owed. If you have evidence of line management request and approval that they wanted the overtime worked, it will strengthen your case.
If you get absolutely no response or comfort factor they intend to remunerate you for monies owed, when you did the overtime in good faith, then your recourse could be to take your claim to Tribunal, but I recommend you only do so if your relationship with the company has completely broken down, as it can sour the relationship if you still want to work there.
Ideally, formalising the matter in a single correspondence could hint to them it is a claim you could take to Tribunal and they will want to avert that risk.
I would also find out, now you have moved to the new company, what is their approved process for overtime, including committed dates by when overtime amounts get settled, so that you can prevent this happenin in future.