I mean your case is simple - you agreed to a £4 charge, they charged £104, then they admitted that £100 was charged in error and agreed to refund, but have not.

What you could also do is a GDPR request for the recording of the call when they agreed to refund. The other option is to complain in writing, wait 8 weeks, and then go to alternative dispute resolution, which, on reflection, is probably the best way of handling this. If you play the call back to the arbitrator I don't see what could go wrong. Details:

https://www.caa.co.uk/passengers/res...te-resolution/