Property re-negotiation price on Land not owned by owner

So im currently in a process renegotiation for a property as we had some findings on our RICs survey. When I was looking at the title plan, it shows a narrow strip of land at the rear that is not owned by the owner. I read the conveyance and apparently there used to be a walkway behind the property that was in use. Now the garden is physically enclosed in that area (neighbours also have done this). I'm assuming that a previous owner some decades back may have extended the rear garden fences to envelop the "abandoned walkway" and many other neighbours followed suit. However the title plans still show the original boundary line. The estate agents, sellers and my solicitors were not aware of this but I noticed it because we were wondering if this was a mistake on the title plan until we read the conveyance and it showed on there an old walkway that used to be there some 100 years ago. My question is could i Re-negotiate the price of this piece of land that is not owned by them as I'm purchasing less land than what was shown? How much can I ask for? I would say it looks to be about 5% of land that is not owned. I will try get an indemnity insurance to cover us as well and an ST1 form from seller to state they have had full ownership of land for past 12 years so we can claim adverse possession later on.