UK Citizens Thai bank account funds after death.

My Mothers Husband died in Thailand earlier this year. Both of them are UK citizens. I travelled to Thailand to collect the Death Certificate to bring it back to the UK where my mother lives. They were both still married at the time of death but separated for a number of years. He remained in Thailand on a retirement Visa until his death. My mother returned to the UK some years ago after they separated. Unfortunately his UK passport and bank books were lost. We believe he had funds in Bangkok Bank and my mother would like to know how we should proceed in trying to retrieve any balances that are still in his account. He did not make a will. 
Many thanks