I used to work in a customer service department, and I can share an insiders perspective. But these are only my thoughts on the MyHobbyStore issue, every company is different.
If writing to customer service does not work, then go higher. It may be CS is a small team with a bad CS manager who is overworked, underpaid or both. It may not do any good if the company generally has a problem. But if the problem sits in the CS managers chair, then escalating it can help.
Check the Website for contacts. Or if it is a PLC check for reports on the internet. Companies house will have a list of directors.
Address your letters - FAO [name] CEO
Writing a letter blasting customer service may make you feel better, but it won't do much apart from get peoples backs up.
It would be more useful to express your disapointment and annoyance (politely) and explain that the company is now starting to get a bad reputaion lakeside and on forums. And that this must be damaging the companies reputation and ultimately thier business. That should get his/her attention.
Then outline briefly your recent problems in dealing with his company. And politely suggest that there may be a need to provide training in customer care.
Avoid blaming of slagging people off. It is never that effective, people just lie to you more politely.