‘Ascent” is the title for the National Youth Orchestra’s current tour, and the dominant theme is mountaineering, at once taken literally and as a metaphor for artistic aspiration and striving. Strauss’s Alpine Symphony, pitting man against the impersonal magnificence of nature, is the main work, to which the evening’s curtain raiser, a new piece by Dani Howard, also called Ascent, forms a response. Separating them is Smetana’s great depiction of the river Vltava from Má Vlast, marking the bicentenary of the composer’s birth which falls this year.