As previously said find a good house conveyancing solicitor and get the covenant scrutenised, it may have been badly drafted.

See if the council have a standard rate for covenant removal or did they just make this one up.
Regardless of the outcome of the above, make the council an offer, on the assumption theirs was a first offer.

Covenants are there to protect either the landowner, current or previous or the adjacent land and or property owners, if it was next door to you trying this you would probably be gatefull they couldn't just build and make a Substantial profit, like your mother will, even if she has to pay £15k.

Land = Free
Build cost = £50k
Covenant = £15k
New house value = £120k

Seems to be a £55k profit there.

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