Train reservation

We would like to reserve 2 sleeping berths for journey from Bucharest to Sofia (for May 13) on EC383 and back from Sofia to Bucharest (for May 15) on EC382. We hold global Eurail Pass (first class). We are unable to book on line, nor my travel agents in India are able to book. Please advise.
. Can you book through email?, If not,
. Can we book at the Eurail office in London (on April 29), or Zagreb (on May 04), or Budapest (on May 9), or Singhisoara (on May 10), or Brasov ( on May 11).

We are seniors and can not Tavel without reservation.