In most cases the Land Registry official plan won't give precise boundary indications.
Have a look at Land Registry Practice Guide 40 (Boundaries).
https://www.gov.uk/government/publications/land-registry-plans-boundaries/land-registry-plans-boundaries-practice-guide-40-supplement-3
You may find that the old pre-registration deeds (if they're still available) have useful measurements. However, if after measurement somebody finds that they actually own a few feet of land on the other side of the fence, now within their neighbour's garden, there's usually very little that they can do about it if their neighbour has used the land for 10 or 12 years (depending if it's registered) because the neighbour has adverse possession.
After 10 years’ adverse possession, the "squatter" will be entitled to apply to be registered as proprietor in place of the registered proprietor of the land.
I doubt that occasional maintenance of a hedge will be sufficient to obtain a possessory title.
"Adverse possession requires factual possession of the land, with the necessary intention to possess and without the owner’s consent."
https://www.gov.uk/government/publications/adverse-possession-of-registered-land/practice-guide-4-adverse-possession-of-registered-land
[The Practice Guide has already been posted by a previous poster.]
NoRush, your comment that "We don't own the hedge" is pertinent to the issue, as is making enquiries as to ownership. Likewise, trying to bill a third party for your costs of maintaining the hedge over the last 20+ years would be tantamount to acknowledging their ownership.
I suspect that on paper (at least) it may well be owned by the developer of the housing estate. I think that you may be able to defeat the claim of the local rich guy, but I doubt that you'll get possessory title either.
You could try going to the local planning office and inspect the file for the development of the housing estate. The more recent the development, the more likely that there are likely to be site plans etc. showing the extent of the developer's ownership at the time of the planning application.
Obtaining a copy from the Land Registry of the relevant transfer to the nearest house on the development may also show ownership of the land on which the hedge is located.