My advice to prospective customers: Avoid the hassle and find a reputable provider.
My advice is to check any prospective supplier is a member of an Ofcom approved ADR. This is a requirement of any provider, even simple, systemless resellers.
If a provider is riding shotgun over Ofcom requirements and you come up with problems like they won't issue a MAC, or they cut you off because they don't like what you are posting on forums. You haven't got anywhere to go. These guys are already outside the rules.
You can check providers that comply with Ofcom ADR requirements here.
http://consumers.ofcom.org.uk/2009/12/adr-schemes/
If you are using a small provider or systemless reseller, I would advise checking membership regularly as some providers leave the ISPA and ADR schemes and this may be a warning sign as the slice of cake the small guys occupy is shrinking daily.