There's a difference between these and buying units in a cash fund.
A SIPP bank account is an actual bank account with Lloyds or RBS or whoever, and you get statements that look just like the ones I get, and the account is in the name of the SIPP.
Buying units in a cash fund is a bit like buying units in the FTSE 100 tracker fund, except that you expect the units to remain at a constant value of £1 per unit, and you expect income from the fund in a similar order of magnitude to bank interest. This is the type of pension cash investment that has lost money recently.