The AAIB special bulletin states that the highest recorded altitude was 2600' but that then radar return was not continuous. This would appear to leave open the question as to whether the aircraft went higher after this reading, or had been higher before it, but thinking about it would that not make the situation worse?
If the aircraft was heading upwards when the 2600' and 100kt were recorded, then it would suggest that it was even slower over whatever the eventual "top" was. If it was on its way down when 2600' and 100kt were recorded, this too would point to it having been slower over whatever the top was.
Is this logic too simplistic?