SIPPs: Why are interest rates on SIPP deposit accounts falling?

The interest rates on SIPP deposit accounts have been falling for a few weeks now, although the downward trend has gathered momentum over the past few days. We all know that Bank of England base rate has been stuck at 0.5% for over three years now, but just what is causing the recent reductions in SIPP deposit account interest rates? Swap rates Need advice on your SIPP?...