I've been tracing the witnesses to the Will.
The first is Colonel Stewart Francis Newcombe
https://en.wikipedia.org/wiki/S._F._Newcombe A friend of T. E. Lawrence. He was in hiding in Constantinople 1917 having escaped from a POW camp with the help of French/Syrian women whom he later marries. Frew might have met the colonel at that time in Constantinople but Frew was himself in prison in July 1917 and I don't know when he was released.
The colonel gives his address as the United Service Club which was a London gentleman's club for high ranking military officers. It was also the most expensive club with a building on Pall Mall.
The second signature, however I can't make out. The last section is presumably his/her rank or profession but I can't make that out either.
Can anyone help?