Please, I beg you not to do this. Buy it as is, do not start a boundary dispute.
I think people are under the impression that the law covers you it doesn't. It merely comes down to who has the biggest balls.
You need to look at GardenLaw website forums (boundaries, fences, hedges) to see that sometimes these things go on for decades, I really wish I was kidding you. It would be very rare for a neighbour to say, yes of course, here is the 3ft of land, sorry about that.
These things turn personal. You were happy with the house, plus happy with the extension. Why would you want to make yourself potentially, and very likely, unhappy? Measurements taken from hedges move, and the fact that your deeds have measurements on tells me the house is probably very old.
Please read this from a conveyancer on Gardenlaw called put your tape measure away
And secondly, lots of legal matters are covered by your house insurance, however, boundary disputes are not. Because they cost a hell of a lot of money. It is easy for an architect or a solicitor to tell you it is your land, but they do not have to live next door to the person.
Plus unless this is your forever house, you would have problems selling in the future because no-one in their right mind would buy a house with a boundary dispute so you are usually stuck.
I do have experience of this, not directly, but was out with a friend when we were called dickheads by the lovely next door neighbour to my friend. So it doesn't just happen at home. My friend said there is nothing worse than dreading going home. They now rent that property out and neighbour is as nice as pie to the tenants. But still calls my mate wanker when she sees her.