Hi Crispin
No problem, also just to clarify the last point regarding a burndown chart for each SBI. This doesn't have to be on separate graphs. Each SBI burndown chart can be overlayed upon each other. If the burndown chart becomes too busy (lots of SBIs) then you could perhaps produce 3 sets of burndown charts with overlaid graphs. The fist chart showing SBIs that were delivered and didn't deviate much from the expected velocity (Good SBIs). The second being SBIs that were delivered but jumped all over the graph in a volatile way (OK SBIs). The third being for SBIs that were not delivered (Bad SBIs).
Regards
Rob