Just a thought, but I can see a logic which might explain the garage's stance. If the motor/pump had failed, then that must be a warranty item, no question. However, if the motor runs then it must be something else, probably a blockage somewhere. The garage might suspect that an incorrect or dirty fluid has been used (eg washing-up liquid) which has gummed up the filters or NR valves or the nozzle. That would be user error, and wouldn't be covered by warranty. If the filter or NR valves are integral with the pump, that would explain why they want to replace it. If that's the case though they really ought to explain it to you.