Do you have to pay tax (income or capital gains) on the profit from a house sale?

[cassandra164]

My mum and dad are hoping to sell their house and buy a new one. They hope to have about £30-£40k left over once they have made their new purchase. They are concerned that they have to pay some form of tax on this profit. Is it considered 'unearned income'? They are both retired and receive private/work pensions and my dad receives incapacity benefit. Any advice please?