I agree with Henrya,except that the Serotine can cause a similar sound
Have you tried looking high in the sky (where the swallows and swifts fly)
I have a Noctule that turns up around 19:45 hrs. long narrow wings fast
flies high Serotine a little smaller chunkier but similar flight
If I cannot pin a call down I ring around my Bat Group or go to
Warwickshire Bat Group to check
You do seem to have the technique off pat though just remember
a change in terrain will alter a call eg open parkland tall trees