Clearing drains - responsibility

Maintenance

I have usually managed this HMO myself, but I'm now using an agent for full management, they have advised that it's not the tenant's responsibility to clear the drains of moss. It is a student let, the moss is coming off the roof, we cleared the drain in September, but the tenants are complaining they have had to do it again only three months later. I had always thought tenants had the responsibility of keeping the rainwater drains clear of debris as it's at ground level and at certain times of year you do get debris. This is difficult to solve as the moss is coming down the drainpipe from the gutter so a cover would not work. Reaching the roof needs scaffolding. Any thoughts or advice I would be grateful.