EXISTING FENCE

A wise precaution before the installation of any fence is to liaise with neighbour[s] and agree the fence line.
If there's doubt, refer to your deeds and again consult with neighbour[s]. If you are very unfortunate, it may need the services of a qualified surveyor to peg out the true line.

It is not the fence installer's responsibility to determine the fence line and unless he is advised in writing to the contrary, he is perfectly entitled to assume a replacement fence must lie on the line of the existing.