You stated that the land was left to you in the will - in which case, the value of the land is the that at the date of the death ie the value used for probate - this is the 'cost' to you.
The next section is copied from the Which? web site
Home example
For example, you inherit your father’s home when he dies in May 2008.
At the date of death, it is worth £200,000.
You sell it six months later for £205,000 and can deduct selling costs of £3,000.
You have made a gain of £205,000 - £200,000 - £3,000 = £2,000
The value for selling is the actual amount in the sale (assuming that it is a 'real' sale - ie. not an artificially low price for example when perhaps selling to a relative of yours)
The deposit is not a sale - the sale is the complete price for the land including the deposit and is effective when ownership is transferred.
CG allowance cannot be carried forward.