As Collaborate said:
"What is on the ground is much more a reliable guide than the title plan, which is usually based off old OS maps."
Especially if, as you say, this has been in place since the 1990s.
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"We’re talking about a strip of building about 60cm wide so if only a general boundary, how are we to know if it is or isn’t on the boundary line?"
At first I thought maybe you were talking about a couple of metres. But the point you raise is central to the whole thing about Land Registry plans. these plans just show the general outlines of the property.
Land registry plans are typically at 1:1250. What this means is that one metre on the plan represents 1,250 metres on the ground.
Or, more pertinently to your situation, one centimetre on the plan represents 12.5 metres on the ground.
So if you are talking about 60cm on the ground then that is shown by just less than 0.05 cm (half of a millimetre) on the plan - and if you (or your buyer) want to try and measure that, then good luck to you.
"I phoned Land Registry and they said they normally allow a 1m give or take either way"
On a typical plan, 1m is represented by just less than 1mm on the plan.
Your buyers appear to be demanding that you get the boundary of the property altered on the plan by less than one half of a millimetre.
Just putting things into context here.