(Edited to cut time between calls)
It was flying around the Warren fairly high up then moved off North-west or poss onto the Saltmarsh, but I lost it to view. At the time I thought it sounded a bit different to the usual Rock Pipits so pointed my mic at it recorded a few calls and thought nothing more of it.
Have been looking at it in a bit more detail today now I'm back in Birmingham with my laptop and this is what I've found.
This is what a sonogram (edited to cut time between calls) looks like
That little purple line is what I reckon is roughly in the middle of the calls. I then did the same thing with 6 Water Pipit and 6 Rock Pipit calls taken from Xeno-canto. I made sure to choose call types that were the same as the Spurn bird (flight calls) and that they were the same race i.e. petrosus Rock Pipits from Britain or France and spinoletta Water Pipits from Europe (Poland, Italy and Germany).
Rock Pipits are on the left and Water Pipits on the right
I then worked out the average level (kHz) from the Rock and Water Pipits calls.
Rock Pipits came out at 6.491 kHz (range 6.682-6.109) and Water Pipits at 6.038 kHz (range 6.395-5.727).
The Spurn pipit = 6.095, does this mean its a Water Pipit?
I don't know!
The lowest a Rock Pipit showed up in this test was 6.109 kHz in my very crude test, so does this put the Spurn Pipit outside the range of Rock Pipit and make it a Water?
In Catching the Bug (Sound Approach book) they show a sonogram of Rock and a sonogram of Water. It shows the shape of the calls in the sonogram to be quite different! The Rock Pipit has a steadily rising shape whereas the Water is a lot more up and down or 'rising with modulations' as they say in the book! It seems that from the calls above they vary massively! So wouldn't fancy commenting on the shape of the Spurn pipits call!
Obviously its a small sample size, but shows that the Spurn pipit is towards the lower end of Rock/Water Pipit calls.
Be interested to hear from a few people that know a lot more than the very little that I do on these birds calls!