Even if you are tax resident in Spain you may well still be domiciled in the UK and your estate liable for UK IHT. You need to have almost no ties to the UK to convince HMRC that you are not UK domiciled. The OP has not stated whether his mother was even tax resident in Spain.

I was just thinking the same thing Fred - If the OP is confused by the language being used here - when we say "resident" we mean legally resident, with a green residencia form/card OR TIE card, submitting yearly tax returns etc.

When the OP says resident, hes probably talking about where she lived ie; where she resided at the time of her passing. "where she was a resident"

As she spent 50% of her time this might be more complicated than everyones thinking.

A lawyer is the only way. be prepared to pay.