[KEPOS]

Well, then I stand corrected. I always thought the prototype suffered a prop strike while taxiing which destroyed the engine.
Edit- Having looked at the video on Youtube I see that the first attempted test flight was a failure because of the prop strike, but that a later test flight was successful. Does any actual data exist from this flight?

There is data on all three test flights. However, it's difficuilt to tell what an aircraft's ultimate handling capabilities are from just the first couple of flights, since the first number of flights for any new aircraft are always very conservative and very carefully executed, with the test pilot recording every little quirk found during these flights, no matter how small. The data is then conveyed to the engineers, who will make adjustments to the aircraft as seen fit. So, you’re right in that regard, any real FM would still be a bit of a guess. According to the design specs the Sinden was expected to be highly maneuverable, though.