Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
I am sorry to hear of your loss, and in answer to your question, Yes - I am afraid you will, and I would contact HMRc to asl them to issue the appropriate years self assessment tax retirn on which you will have to declare ALL mums income for5 that tax year (so any pensions, and sources of income along with any savings and investments along with this gain)
However, so we can be sure that you even need to go to all this fuss (as there is an exwmption allowance each year so there may be no tax charges arising) can you advise
1) the date this land was sold and how much for
2) How long the land had been owned for and its purchase price
3) Whether just in Mums name
4) Whats cost 9such as legal or estate agent costs there were to buy and sell)
I can then advise further