Hello and welcome to the site. Thank you for your question.If the monies were your mother's (she inherited this sum from her mother) irrespective of the fact that you had a joint account, you should include this sum as her asset when completing IHT205.
More information on this can be found on Page 17 of IHT206-2011 (notes)
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.
Thank you. Do I need to do anything myself to declare or complete any form for my own tax purposes or because it is below the IHT threshold it is not required? Karen
Karen, thank you for your reply.You don't have to report any sums you have inherited to HMRC on any form for your own tax purposes.I hope this is helpful.