The soundtracks were recorded off-air
from the BBC transmissions in the 60's &/or 70's, & are said to be of good quality even though probably unrestored.

The dubs have been synched to the already complete visual materials of the series
ie to digital copies of the film prints.

Only one episode is still without its English language dub:
Episode 3 - 'Suspicion falls on Andrej'

But that episode is available with great English subtitles, skillfuly translated by an enthusiast some years ago ( & who put out a fantastic subtitled edition of all the episodes at that time )

The ( unrelated ) person/people who've made available the 'new' version with the unearthed dubbed-in-English soundtracks have included the subtitled episode 3 as part of the new set.

Search internet for the website:
thewhitehorses.co.uk
& that's where it's available from.

I heard about this a few weeks ago & I think it's probably been available for about 6 months or so, no more than a year at most.
I've just ordered a set.

Anyway: It's great news.
Quite likely that you're already aware of this new dvd set Iain, & Little Gems website visitors here - but worth a mention in case people haven't yet heard about it.