Ah yes - the 'penny' has finally dropped. A combination of surprise that turbulence could 'show' up as badly as that, plus the MAG-TRK being 'in the wrong place' [to a steam pilot
] ie up the page, and the[my] 'FMC' bug in fact being an MCP bug - was what the trouble was all about. Sorted. Many thanks - both of you.
CB