It's clearly intended as a continuation. You remember the 1978 series, and
perhaps the 1980 series, both of which had Douglas Adams directly writing the
scripts, often still doing so astonishingly-close to transmission. They only
loosely correlate to the first two books, often having quite different plots.
The third through fifth books were turned into three more radio series by Dirk
Maggs in the mid-2000s with varying levels of success. Series three is the
worst, perhaps because Maggs was still getting the hang of it, but still pretty
decent. An obvious but well-executed change to the plot is to replace digital
watches with mobile phone ringtones.
All of these have been released by the BBC on CD, which you can buy from the
usual culprits. CD releases tend to end up in the DRM-infested shoddily-encoded
download/streaming services as well if you want to just set fire to your money.
The best-value way is to buy the five series boxset (it's about sixty eurobucks
here), even if you only want the first two series since it costs the same and
throws in a bonus disc of rare material. Well, the absolute best-value is
bittorrent, but that's between you and your sense of ethics.
This sixth series will be released on CD once they've broadcast all the
episodes. It's probably one for the completists.