Just for info --- we were contacted by a firm of Heir Hunters a few years ago. I checked on them, like you, and they were legit.
They didn't give me a name BUT - they had contacted me firstly via Ancestry, enquiring initially about my husband's grandparents. My tree was private so I asked this person how they thought they were related before I opened the tree. At that they revealed they were Heir Hunters and gave me the company name.
Hence knowing the surname they were looking at - I searched Bona Vacantia and discovered that the eldest sister, unmarried -- of my father in law, was listed. No amounts were shown.
We decided to go it alone, as it was a big family and we contacted almost 30 other descendants. We had to send several records to the Gov Dept -- then they would send those back and ask for the next lot and so on. It was as though they were checking and ticking off that evidence before proceeding to ask for the next batch of evidence. Only one member of the family applied (otherwise it would have been a nightmare).
It was rather laborious I have to say -- and a very small estate. However it was so interesting to see how the system worked. We did split it between all my husbands brothers, sisters and cousins. Quite a lot of admin was involved!