I thought I'd chip in as well. I tried 2 approaches, I emailed the regular customer services dept threatening the ombudsman and possible further action and about 5 days later I sent a letter by recorded delivery to the MD (Thank you very much for postal the address!) threatening small claims court to retrieve my money plus interest.
I subsequently received an email saying that my payment had been processed and I would be receiving the cashback soon. This payment has just turned up in my account a week later!
I received the email the same day that I posted the letter to the MD so it must have been the email that prompted the response. This is really good news! I just have to repeat this 3 more times to claim my subsequent cashback payments!!!
After this experience I will never deal with mobileshop again and will warn anyone any everyone I can off them. I think I will just go direct to the networks in future and avoid cashback deals after being stung by this.
Good job everyone who contributed to this success and good luck to all the others who try this route. It appears that mobileshop are only processing the claims that they have to to stop people taking further action. This makes me wonder if the business is going down and they're just trying to keep afloat as long as possible.
Let us know how your all claims progress. We could always organise a flashmob at the HQ. Pitchfork anyone? ;)