Hello and thank you for your question. I will be very pleased to assist you. I'm a practicing lawyer in England with over 10 years experience.
Do you know what kind of sewer the pipe is please - e.g. a highway sewer or sewer carrying residential waste for example? If the latter, presumably from what you say the sewer serves more than one property?
Thanks. On that basis maintenance of the sewer rests with the sewage undertaker which from what you say is Yorkshire Water.
Following a change in the law a few years ago under the Transfer of Private Sewer Regulations, in addition to publicly adopted sewers, sewage undertakers are now also responsible for any private sewers which serve more than one property
accordingly, you may wish to notify the sewerage undertaker of the situation in the event that they wish to take action. If the tree was planted on your land the tree itself is your responsibility however you only liable for any damage the tree causes if it is or was dangerous and you fail to take action.
if the tree was planted in your land and has fallen over but is not likely to cause any further damage to pursue when the other property, then you can consider simply leaving well alone. If however the tree is likely to cause damage and you failed to take action and subsequently the tree does cause damage to pursue, you may find that Yorkshire water seek to claim damages from you for negligence
is there anything above I can clarify for you?
Does the above answer all your questions or is there anything I can clarify or help with any further?