Simple answer is no, you do not need to invest the cash held in your SIPP.
In a Self-Invested Personal Pension (SIPP), you have the option to hold your pension funds in cash. Cash within a SIPP is essentially uninvested funds that are held in a cash account within your SIPP, however clients must be aware it doesn't earn interest or investment returns in itself, but it provides flexibility for future investment decisions.
If you are using a DIY portfolio within your SIPP and have the Autoinvest feature turned off, the cash balance will remain uninvested until you place your orders, or use either the Autoinvest or Rebalance feature.
On the other hand, a Managed portfolio within your SIPP will work the same as with all our managed accounts: your cash monies will be invested in the next available trading window.