The booklet doesn't have the usual technical info, since the BFI would have worked directly from Pathé's 4K restoration and wouldn't have needed to do very much (if anything) themselves, short of encoding it in such a way as to bring out its virtues when reduced to 1080p.
However, a bit of digging turned up a very detailed account of the 4K restoration, from the Cannes press book:
The restored print was created by digitizing the negative image in 4K. Preliminary tests in 2K and 4K revealed that only 4K digitization would be able to translate the contours of the image, the refined skin tones and the light and wonderfully diffuse glow of films from the time, obtained through the filters used by the film’s two directors of photography, Geoffrey Unsworth and Ghislain Cloquet.
The restoration was carried out entirely on these raw 4K files obtained by digitizing the negative and, in line with the techniques used by the Eclair Group’s heritage department, color grading was carried out in parallel so it could be applied to the restored images at the very end of the restoration process.
Several shots and sequences, mainly involving special effects created on an internegative (lower quality than the original negative), presented several difficulties such as high instability, different stains resulting from processing the film, and grading problems involving an inconsistency between the left and right sides of the frame.
For example, the shot of Tess in the winter landscape in reel eight was damaged and replaced by a third generation internegative made from the interpositive. Imperfect definition combined with a significant increase in grain made it difficult to edit this shot into the rest of the sequence. Eclair thus started again with the interpositive – gaining one generation of the image – and used several different restoration tools to reduce the grain of the shot and increase definition. Finally, the grader applied a series of masks to balance out the left and right sides of the frame.
In another sequence, when Tess arrives in the village and changes her shoes in front of the wayside cross, it was necessary to start again, just using the interpositive, for a few damaged shots. These shots were linked together with the rest of the sequence (contrast and definition).
Major work was carried out on the sequence of the dripping cheese in the dairy, which had lost all of its density when it was accidentally exposed to X-rays during the shoot.
In terms of grading, the teams were lucky enough to have at their disposal Roman Polanski’s personal copy for reference, printed at the time of the film’s release and perfectly preserved. The aim was to respect the original grading whilst adding a few improvements here and there in terms of links (contrast and color), made possible by modern digital grading techniques.
To respect the original anamorphic widescreen, the 2.35 format of the scans was adapted to 2.39 digital projection to preserve the height of the image (on the edge of the vignetting). Just a few shots were reframed during grading to eliminate a few flares on the edge of the frame, particularly visible on the original copies.
Sound restoration was carried out by L.E. Diapason. TESS was one of the first films to use Dolby Stereo, the format that would revolutionize cinema by introducing multichannel sound to the majority of theaters. Although it is likely that the sound was mixed using equipment poorly adapted to multichannel sound, the film’s soundtrack was already exploring the possibilities offered by the format, mainly in terms of the lavish treatment of music and the extraordinary work carried out on the atmosphere which offers a rare depth and quality.
The restoration process had to respect this work and recreate it in all its detail. To do this, L.E. Diapason used 35mm four-track magnetic sources, which are unusual in that they contain a very diverse sound. It very soon became apparent that not all the reels had been mixed in the same studio.
Moreover, the magnetic elements hadn’t aged very well and some reels were noticeably deficient in high tones while others were in good condition. Aside from the usual work to alleviate the effects of ageing, the main task was to bring consistency to a source that was anything but consistent, whilst preserving the integrity of the mixing and its more delicate elements.
The advice of Hervé de Luze, Frédéric Moreau, Olivier Chiavassa, Philippe Tourret and Raymond Terrentin who worked on the photography during post-production was extremely valuable. The sound restoration also owes a great deal to the expertise of Gérard Lamps and the advice he gave to L.E. Diapason.
All of this is completely borne out by what I've seen - it appears to be absolutely faithful to the film, and Polanski was apparently thrilled by the end results.