Land registry problem between freehold and leasehold, only spotted 20 years after the sale - what are the best steps to take to resolve?

My dad has recently come to sell a house they bought 20 years ago, and when speaking to a solicitor (not the solicitor who dealt with the original purchase), the solicitor has pointed out that whilst my dad is registered as the land registry as the freeholder, the leaseholder is listed as someone else entirely - we think the family members who owned the property 20 years ago.

For some context, there's three people on the leasehold, all with the same surname, and they were registered as the owners in Jan 2006, we are surmising through inheritance following the passing of their parents. My dad then purchased the property and is registered on the freehold as owner from April 2006.

The price paid for the freehold is the full price of the house, this is obviously way too high to simply be a freehold purchase. Indeed it looks like someone, somewhere, as simply forgotten to update the leasehold, or merge the two titles.

So, technically speaking, my dad owns the freehold, but not the leasehold, which not only means he cannot sell it as easily as he'd like until it's resolved, but also that technically they don't 'own' the house that's on this land, which belongs to the leaseholder...which is not my dad!

So my question is, has anyone had this happen to them, and/or know of the simplest solution? Importantly, has anyone had it happen and it turn out badly for them?

Due to the length of time, we're not even sure who the solicitors were that did the conveyancing, though I'm sure we'll be able to find out, but ideally we'd just have our current solicitor apply to merge the titles, but that will surely involve the actual registered owners, and what happens if they decide they want to pretend they always owned it?

A mess for sure, that I'm hoping has an easy solution. I'd welcome any advice :)