Just talking to DH about this as he is a builder, and the measurement is taken from the centre of the post, not where the palings are, so you need to check that firstly and then see if it is worth following up further. You need to be sure how far out it is according to the title line, not just where the old fence was. If there is a further dispute, the builder would be liable but it is the owners responsibility to show where the new fence is to go. You might need to even find out if the old fence was in the right spot because it is possible they have moved it in accordance with the title line. Really tricky hun, once you talk to them you may also need to get a surveyor out to measure it out accurately according to the title line, because if it is out by 100mm you would want it moved.