I have been a fan of the Quatermass universe since the early 1960's which I first saw the second feature film (Hammer production of "Quatermass II", also titled "Enemy From Space") on television and got the bejeezus scared out of me. The character is the creation of Nigel Kneal and the two original theatrical releases ("The Creeping Unknown"...theatrical title of the "Quatermass Experiment" story in the US and "Quatermass II: Enemy From Space") were brilliantly directed by Val Guest, despite the limitations of special effects of the 1950's and the obviously limited production budgets.
Recently I obtained the original television versions of those two stories as well as the original 1950's television version of the third Quatermass story (released in a theatrical version in the late 1960's under the title "Twenty Million Years To Earth", but originally titled "Quatermass and the Pit" and produced in the 1950's as a limited run series.) I was quite amazed to see the vast differences in the storylines of the tv mini-series versions and the compressed theatrical versions. Generally the tv versions are considerably looser, slower and sometimes tend to go in inexplicable directions, defects nicely corrected in the shorter theatrical versions, most likely for budgetary and running time reasons.
I am also a fan of David Tennant, having completely absorbed his entire Doctor Who canon (you will be missed, David).
If you wish to make a comparison of the way the same story can be approached in what are essentially two very different media (television vs. theatrical film) I would enourage you to obtain not only this release but also the 1956 Hammer Films version of it titled "Creeping Unknown" and starring Brian Donlevy as a very American Quatermass. I think you will find that the 1956 version is much "tighter", much better directed (Val Guest), with a better script and compelling performances. While this "live" 2005 production has its merits, it seems to have been done merely for the point of being done live on televison, a kind of experimental throwback to the very era when the original was produced. In the US we had a similar "live television" production of the political drama "Fail Safe" several years ago, starring Richard Dreyfus and George Clooney. They were both quite interesting but perhaps too expensive or simply tension making for the proudction companies to ever repeat. Indeed, even "Saturday Night Live" is no longer live, strictly speaking, being on a several minute tape delay (to cut out any naughty bits that might be blurted out by over-excited performers in the midst of the hilarity.)
Still, if you are a collector of all things Quatermass, this is a must for your dvd shelf.