Probate help - divorce certificates

My mum was married in 1978 and separated shortly after. I have notified her estranged ex-husband of her passing who says he has remarried and can not remember details of divorce. Date court etc he is a stranger to me and not massively helpful local court has no record I have the option to apply to central courts in London to search all courts at a cost of £65 per decade upfront fee unsure if refund if no results and I need to search 4 periods.

Online form simply asks if there is a surviving partner which I can say yes or no. I’m debating if online is the better option and they do their own checks. Or I will need this certificate at some point regardless.