The potential problem is that (unless it has a screwed down lid - unlikely on a gravity one), in the event of rainwater surge, it lifts the cover and sewage goes everywhere in your garden. You (or your solicitor) can ask an enquiry question of the sellers to see if this has ever happened.
Otherwise, there will be an easement in favour of the utility company entitling them the passage of this pipe, and there's not too much more to be said. Except that you cannot build (new build, that is) on land within X metres of these things. X is typically 5 metres, but different utility companies vary their rules. Is there any danger of it fouling up (not pun intended) any future plans you might have for building an extension to the back of the property?