At about the time that Duty Free was being scrapped in the EU UK airlines got a bit worried that airports would put up their fees. That, in turn would mean the airlines might (would!) have to put up their fares (which they didn't want to be seen doing).
So, the decision was made to take these fees (or at least those applying to UK airports) out of the fare and show them separately on the ticket. They decided to use the "tax" box. Agents took exception claiming they had been earning commission on that element so why shouldn't they continue to do so (they don't earn commission on taxes). Airlines responded by redefining the tax box as "taxes/fees/charges" but it did no good and they lost in court (mind you, last I heard Ryanair were still holding out againts paying back the commission as ordered by the courts on tickets issued between the change and the injunction).
Anyway, the point is that not everything you see in the tax box is a tax controlled by HM Customs and Excise. In fact, probably the only one you see that they have any control over is the tax with the code GB. The UK airport fee is shown as UB. All(?) the others are levied by non UK entities (usually governments, but not always). Therefore, complaining to Customs and Excise is only part of the solution.
However, it MAY not be the airline that's holding the money. If the ticket was bought through an agent then it could depend on how he/she dealt with the refund. It can also depend on how the ticket was paid for in the first place.
IATA agents can either apply for a refund (in which case they have to wait for the airline to action it) or they can complete the calculation themselves and simply process the refund against issues. Indeed, most of the CRSs now provide an electronic method so the refund is processed as quickly as the issue.
Then we come to the method of payment. If the payment method is anything other than a credit card the agent might wait until the refund has appeared on his billing and the payment cycle has completed (with the refund being deducted) before making the refund. This shouldn't take more than 6 weeks but I can accept it might be as much as 10 weeks before that completes.
If payment was by credit card it depends on who was the merchant. If the bill appeared on your card under the name of the agent you bought the ticket from then (daft though this may sound) he has reported it to the airline as a cash sale and the previous paragraph applies. Of course, in both cases you are then in the hands of the agents as to how long it takes after they have realised they've got the money back before they process it.
If the bill on your card appeared under the name of the airline then the agent returns the ticket there. Now you're back to the manner in which the agent returns the refund. If the agent calculates the refund and simply submits it then you're dependent on how the airline processes the billing to the card. If he asks for the refund then you wait while the airline calculates the value and returns it to the agent. Then you wait while the agent subimits it back to the airline again at which point you're once again back to the airline and how they process the billing to the card.
Horrible isn't it?