It is all going to depend on the survey. That is when a surveyor comes in, its down to that report and that is expert evidence a Court will require.
Criminal Damage is straight forward. Under Section 1 of the Criminal Damage Act 1971 states:
(1)A person who without lawful excuse destroys or damages any property belonging to another intending to destroy or damage any such property or being reckless as to whether any such property would be destroyed or damaged shall be guilty of an offence.
(2)A person who without lawful excuse destroys or damages any property, whether belonging to himself or another—
(a)intending to destroy or damage any property or being reckless as to whether any property would be destroyed or damaged; and
(b)intending by the destruction or damage to endanger the life of another or being reckless as to whether the life of another would be thereby endangered;
shall be guilty of an offence.
(3)An offence committed under this section by destroying or damaging property by fire shall be charged as arson.
If the Police prosecute, your neighbour could face a range of sentences from a conditional discharge to prison.
Does that clarify? Alex