Thanks for your question - I am Sam and I am one of the UK tax experts here on Just Answer.
As long as this property was your main residence up to the date of sale, or within 18 months (if you sold after 06/04/2014 and if prior to 06/04/2014 - then there is the last 36 months exempt) of having moved out of it, then you are correct you have no tax liability.
As far as the gift is concerned, this will be a consideration for Inheritance tax only, if
1) You do not live for more than 7 years and
2) Your total estate including the gift is more than £325,000
If you do live for ore than 7 years OR your estate (which includes this gift) is less than £325,000 then there is no Inheritance tax liability to consider either.
And your daughter and son in law, do not create a tax liability on receipt of this money, as gifts made, and not liable to income tax.
So they have no further concerns on the receipt of this lump sum.
Feel free to ask any follow up questions on the above information