gingko wrote:

Does anyone know what type of root structure a common holy tree has?

Holly roots are fiberous but do not go very deep any distance from the
trunk, are not invasive, but as with most trees their surface roots
extend some distance past the tree's drip line.

We've just moved and within 12 hours of moving in our neighbours had
sent round their friendly tree surgeon to enquire if they could pay for
our Holly to be removed. It is close to their property and apparently
within 3 m of a drain.

Ten feet away is a pretty safe distance... seems to me like it's their
drain that's too near your property, they should move it.

I know they are a slow growing species and this
one is approx 25ft high. I obviously don't want their property to be
damaged but didn't know if root system is deep and invasive or shallow
and far reaching.

It's very strange that they didn't ask the last owner to remove that
tree for some 25 years... probably did (many times) and were refused
(each time), probably a long standing feud over that tree. Holly
being a dense evergreen offers excellent privacy, I'd not remove a
healthy holly. Your neighbor can easily dig down on their side and
cut whatever few roots may be encroaching on their drain... your tree
won't suffer from some small feeder root pruning and probably will
benefit. They can easily shield their drain from roots from your tree
and their trees with a few dollars worth of aluminum flashing. That
drain business is just a smarmy ploy... odds are they don't like that
tree because it interferes with their snooping... next thing they'll
ask that you remove your window shades. LOL