No expert, but that sounds absolute balderdash to me. Are they saying in ye olden days no one ever unblocked pipes but continually replaced them ? Why don't they have piping to push/break up the blockage, or a vacuum to suck it out ? Probably just leaf debris anyway. Check the contract or get someone knowledgeable to do so, then if you know you are entitled to service threaten them with the ombudsman and/or small claims court, or something. Maybe ask Citizens' Advice ?