I remember that the book said L. On the other hand, two electrical engineers called Edmund Hill at the same time seems unlikely (admittedly not impossible), whereas typographical errors, even on a title page, are not uncommon.

The one Edmund L Hill in the 1901 census is a painter from Cornwall.

The 1911 census has an Edmund Lewin Hill aged 40 in Wales, but infuriatingly gives no occupation; and shows Edmund Rowley Hill living in Henley.

I must find out who is archivist at the I.E.T. (formerly I.E.E.) and see if he/she can clarify things for me.