Not sure this will be much help, but here goes.
We bought a LH upper floor maisonette (one of just two). The freeholder had sold the upper flat to our vendor some years previously, and had apparently disappeared - he hadn’t requested any ground rent (which wasn’t too much) for several years.
It was thought that he’d returned to his native country and his being supposedly absent, caused a lot of problems with the purchase. I eventually found him via a tracing service - he was hiding just a couple of miles away, under another name, because of debts attached to the freehold!
At first he denied any knowledge of himself, but popped up as soon as he thought there was going to be money in it!
I do advise using a specialist solicitor. We started with the High St one who was handling the purchase, but he was useless. The specialist was miles better, but it still took a long time. The owner of the lower m’ette didn’t want to buy her share, so we bought the whole.
The price was worked out officially - there’s a formula, according to length of lease, current price, ground rent, etc. The FHolder can’t just ask what they like. It was v annoying that the FHolder sent someone round to value the place after we’d spent a lot of money doing it up, though - it was pretty much a dump before.
Having asked for no ground rent for years, our FHolder then claimed that he’d submitted invoices and the previous owner hadn’t paid - I’m quite sure he was lying, but by law we got stung for 7 years of it - thankfully not a huge amount.
It was all finalised at a tribunal - well over a year after we’d started the process.
In spite all the hassle and shenanigans, I’m really glad we went ahead and bought it - will save any problems if and when trying to sell in future, and means that we can make changes (e.g. a loft conversion) if we ever want to, or a future owner could, without having to get permission from the FHolder, or pay them a fee.