Complex sentence: In 1921 Watson donated £50,000 to the Leeds General Infirmary, of which he was a board member from 1906 to his death, to replace some of its investments which had to be sold during WWI, presumably to meet running costs.
Split into simpler sentences: In 1921 Watson donated £50,000 to the Leeds General Infirmary, of which he was a board member from 1906 to his death. The monies were used to replace some of its investments which had to be sold during WWI, presumably to meet running costs.