As Therese has said above, please seek legal advice regarding the Calderbank offer, as a lawyer will be able to tell you whether it is a good offer and what you would be set to gain if you went to trial vs the offer presented.
With a Calderbank offer, if you go to court after rejecting the offer and the order is less than what was offered to you in the Calderbank offer you will be liable for the other side's court costs from the date of the Calderbank to the date of the orders.