Unfortunately, the recent collapse of Thomas Cook has had disastrous consequences for their former customers, and you are certainly not alone in attempting to obtain a refund on your monies.
If your debit/credit card lending agreement was with Clydesdale, then they are responsible for refunding you on any chargeback arrangement.
I have never heard of Opodo, but I assume from a quick Google search that they were your travel agent.
You need to check the holiday contract, but I imagine that Opodo charged you for their services as the introduction agent for Thomas Cook, or took a percentage of the price from Thomas Cook. However, the main holiday contract was with Thomas Cook and therefore they are the defendant to this claim.
If all else fails, you will have to approach the liquidators of Thomas Cook with details of your claim against the company. However, you will be an unsecured creditor and therefore second to last in the payout queue when the remaining cash and assets of the company are distributed to other creditors and therefore you will be lucky to receive a few pence in the pound, if anything.