1965: 15800; 2005: 3952, 2006: 3754, 2007: 3711, 2008: 3642, 2009: 3658, 2010: 3672, 2011: 3560, 2012: 3468 (One congregation did not report). 156 congregations: 78 similar, 50 declined, 38 grew in 2012. 63 have less than 10 quota paying members.
Update: others quote 153 congregations with their own base, including places not owned by them, but then 14 fellowships in addition meeting in their own homes or unfixed abodes.
The decline is over 3% a year. I once wrote a piece about Unitarians when 2000 left meeting regionally and staying in touch, assisted by historical money, with the handful of ministers as such training (when in house) at Great Hucklow. This was before the Internet.
However, the existence of such figures does not show that many people in a congregation are more recent, when the older members have been dying off. The average age in Hull, for example, has dropped considerably in the last few years, whilst the raw numbers struggle to maintain themselves and yet do.