FTLs are going to be the primary concern, but looking at that roster it also presents problems for the observers hours.
If each shift is nine hours then over a three week period the observers would work 126 hours, six over their duty time. A quick call to Sussex shows how they fit into this roster:
08:00 - 16:30 (8.5 hrs) for early shifts Mon to Sat
16:00 - 01:00 (9 hrs) for late shifts Mon to Sat
09:00 - 16:30 (7.5 hrs) for early shifts on Sun
16:00 - 23:30 (7.5 hrs) for late shifts on Sun
That totals 120 hours over the three weeks and allows for a half an hour handover. Looking at it I don't like the later shift option to allow for 20 hour coverage across the bases, as the late base would need to work something like this:
10:00 - 18:30 (8.5 hrs) for early shifts Mon to Sat
19:00 - 04:00 (9 hrs) for late shifts Mon to Sat
11:00 - 18:30 (7.5 hrs) for early shifts on Sun
19:00 - 02:30 (7.5 hrs) for late shifts on Sun
That plays havoc with the FTLs as already indicated, and I think most people would want to be on the earlier shifts as there's no working after 01:00!
Why can't they run with a six on four off, six on five off pattern (EEELLLRRREEELLLRRRRR) which needs seven observers and three and half pilots (six shifts not covered in a three week cycle)? This provides 20 hour coverage at each of the bases and requires very little increase in staffing over the nine hour shift pattern above. I know I've said increase when everything is realistically all about decreasing, but why spoil the ship for a ha'porth of tar?