R Smith - I suspect your only option is to apply and provide a certified
copy of the decree absolute
If unavailable for some reason then tracing the previous owner remains an option - I appreciate you state that you are not able to contact them but it's unclear as to why not - have you tried to trace them perhaps but with no luck or have they passed away for example?
If it's the latter then a copy of one of their death certificates is an alternative to submit with form HR4 as well