We get asked this question a lot and so I thought it would be useful to explain it here.
In terms of the way they work there is no difference, the HV30 was manufactured for the trade as a lower cost valve, the whole product is the cartridge which means that once it stops venting you will need to throw it away and purchase a new one. So while it was better value for radiator manufacturers it's a more expensive way of keeping your radiators air free for homeowners.
At first glance you probably won't even notice a difference, but the HV30C has a replaceable cartridge which you can easily remove once the valve stops venting (3 - 5 years). The cost of a new cartridge is relatively inexpensive and so we always urge people to initially invest the extra couple of quid to buy the HV30C as this works out much better for them longer term.
If you do decide to purchase a HV30 elsewhere then please double check that this is the genuine article manufactured by NLB and not a cheap copy because you will not be able to get a replacement should something go wrong unless it is the genuine article. There are several rogue sellers on eBay selling the 'counterfeit' version.
If you have any more questions about this then please do get in touch as we are always happy to help.