This is very odd. The sewer certainly should appear on your title deeds - & on those for all the 30 houses it services. So far as the houses are concerned, you can buy their title deeds from land registry online for a small fee. I suggest you do this for a few & see what info they have. They ought to include an easement to use the sewer running under your land, & may include a clause relating to who should pay costs of maintenance etc.
I can't see how you could be prevented from diverting the sewer (with the agreement of the water authority in relation to the link up point) if you did it at your cost, but I assume this is not what you want. I doubt whether you culd force the 30 householders to pay to divert it, but this may depend on the actual wording of the easement in their deeds.
When you have got some of those deeds you need legal advice. If you find the solicitor who acted on your purchase has been negligent you may well have a claim against them.