Sibelius’s piano music continues to divide opinion, and few commentators, with the notable exception of biographer Erik Tawaststjerna (whose son also recorded it in its entirety for BIS), have made any great claims for it. Even the composer himself was conflicted, once confessing to the impresario Walter Legge that ‘I do not care for the piano – it is an unsatisfying, ungrateful instrument which only one composer, Chopin, has fully succeeded in mastering, and two others, Debussy and Schumann, have come on intimate terms.’ Glenn Gould, on the other hand, was a fan: ‘Sibelius never wrote against the grain of the keyboard. In his piano music everything works, everything sings – but on its own terms.’

Janne Mertanen’s ambitious survey, recorded for Sony Finland but now available online in the UK, fills five CDs, the first of which launches with the Ten Pieces, Op 24, the best known of which is the tenderly nostalgic No 9 in D flat major (much beloved of Cherkassky and others). Elsewhere, the bardic sweep of No 2 in A major reminds us that the intoxicating tone-poem Lemminkäinen and the Maidens of Saari was not long completed. Both the Six Impromptus and Piano Sonata are earlier and show the influence of Karelian folk music (the central Andantino of the latter is an especially fetching creation), but the piano-writing lacks idiomatic fluency and the sonata itself is a disappointingly short-winded affair.

By far the most durable inspiration on disc 2 is to be found in Kyllikki, three darkly expressive pieces that date from around the time of the Second Symphony. I also like the quiet individuality of the Two Rondinos, whereas the Op 34 Bagatelles and Op 40 Pensées lyriques between them serve up a pretty thin brew. Disc 3 brings the most rewarding haul, coupling the Ten Pieces, Op 58, with the three Sonatinas, Op 67. The former display a notably deft demeanour: Nos 1 and 10 (‘Rêverie’ and ‘Sommerlied’) are especially lovely, and Sibelius himself understandably thought very highly of No 5 (‘Des Abends’). The sonatinas (probably Sibelius’s most compelling work for the piano) comprise an agreeably pithy, quirky sequence, their lean textures and pellucid part-writing finding favour with the great Wilhelm Kempff, who once remarked that the First Sonatina was something ‘to polish one’s touch on’.

As for the epigrammatic contents of the remaining two discs, it’s very much a case of feeding from scraps, which is not to say that there isn’t the occasional tasty morsel. The two volumes of miniatures known as ‘Tree Cycle’ and ‘Flower Cycle’, Opp 75 and 85, certainly contain some delightful invention: try No 2 (‘The Solitary Pine’) from the former, sometimes thought of as a self-portrait of the composer. Listen out, too, for the hymn-like tune from the Andante festivo for strings in the first (‘The Village Church’) of the Five Characteristic Impressions, Op 103; and do I detect a suggestion of the Seventh Symphony in the charming No 3 (‘The Oarsman’) from the same collection?

Set down over a period of six years, this must have been a labour of love for the excellent Janne Mertanen (a pupil of Erik T Tawaststjerna and Lazar Berman), who proves a clean-fingered, imaginative and shapely exponent throughout. Recordings are bright and crisp to match. Antti Häyrynen supplies a helpful booklet essay and Sony’s presentation is most attractive. Devoted Sibelians will, I think, enjoy dipping into this enterprising box; the rest should probably approach with an element of caution.