Did you sign a new lease for the 18 months?
The landlord is looking at this form of you breaking your lease (if it is a Fixed term) as a "buy-out" in which case he has the upper hand as it is you who wants to break the lease. Basically he has named his price which you seem to have agreed to.
You should have considered "assigning" the lease in which case, if you found a new tenant, you would have been able to get out of the lease and retain your deposit save for any damage beyond normal wear and tear.
Unfortunately, IMHO your mother does not have a leg to stand on in winning this case - the landlord has done nothing illegal, though taking advantage of your poor knowledge of the law. If you had known the law or been to Threshold and enquired, they would have enlightened you as to how to get out of a Fixed term lease.
However, if you did not sign a new lease you will have a Part 4 lease in which case you could break it by giving the landlord written notice with the required number of days notice, depending on the length of time you have been in the property.
Just a point on the legality of the document you signed - it should have been a Deed of Surrender - if it was not, it may not be valid.