An elderly Sayyid Sunni Hanafi brother has passed away. His sons and other families, who are Shia, will arrange the janazah in their Shia masjid in Chiltern. Can you please find out if we can read our Sunni brother's janazah behind a Shia imam?
بسم الله الرحمن الرحيم
In The Name Of Allah, The Most Gracious, The Most Merciful
There are two types of Shia:
1- Those that hold beliefs that take them out of the fold of Islam. They believe that the Qur’an has been altered, Ali (ra) to be god, accusing Aisha (ra) of committing adultery etc.
2- Those that do not hold beliefs which take them out of the fold of Islam. like believing that Ali (ra) was the rightful first caliph after the death of the Prophet (pbuh) and not Abu Bakr (ra).
The condition of the imam is that he must be a Muslim.  If the imam at the Shia mosque is from the second category, then he will be considered under the fold of Islam, and hence the janazah will be valid.
If the imam is from the first category then he will be considered outside of the fold of Islam, and hence the janazah will not be valid.
If the imam is out of the fold of Islam, is it encouraged for you to speak to the family and explain that as a Sunni, the deceased would have wanted a Sunni janazah. In the case that the family does not accept this, then it is possible for a Sunni imam to perform the janazah at the graveside, in order to give him a correct burial. 
Only Allah knows best
Written by Alimah Nasima Umm Hamza
Checked and approved by Mufti Mohammed Tosir Miah
Darul Ifta Birmingham
 Ibn Abideen, Raddul Muhtaar, Volume 1, Page 548
 Al-Hidayah, Imran Nyazee Translation, Volume 1, Page 274