I've finally got it sorted but it's been an interesting experience. I got an e-mail from them this morning to say they had refunded £14.10.
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You may not be surprised to learn that I did not find this acceptable so replied as follows:
"Dear Ms *******
Thank you for the e-mail confirming refund. The sums do not tally however, as two amounts of £18.80 were taken from my account whereas the refund would appear to be £14.10. Please rectify this immediately. I do not wish to ring your customer service department yet again, nor do I want clarification as to why £4.70 has been retained, I just want a straightforward rectification of the situation., The sooner this is done the sooner I can report the satisfactory conclusion of the matter on the the holiday-related forums of which I am a respected member. "
Passing swiftly over the fact that I am only a member of one forum and may not actually be that much respected on it, this had the desired effect and I have since had another e-mail to apologise and say the extra £4.70 has now been repaid. A reasonably satisfactory conclusion Is therefore hereby reported.