Hi Thomas
I read your post with interest as I'm having a similar problem at the moment. I'm holding cash within a Hargreaves Lansdown SIPP which is earning practically nothing. Long term I intend to invest this money into equities when the market has dropped but for the moment I'm looking to put this money into something within the SIPP where the money will earn something (even if not a lot) before I put it into equities.
Looking through the range of funds within a SIPP you're right there doesn't seem to be traditional saving account type funds such as those offered by building societies. The nearest thing I could find where money market funds. I don't know a lot about them but they seem to guarantee your money as they seem to put your cash into different cash accounts i.e. building societies etc which then earn interest. Check out the range of such money market funds on the following link
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Other than money market funds I think the other option would be to put the money into fixed interest funds (Corporate bonds, government bonds, gilts etc). I don't fully understand the bond market so I can't offer you much advise on this. I'm still reading up on it myself to figure out where to put the cash that I'm holding within my SIPP. There are funds within a SIPP that would allow you to invest into fixed interest funds.
I'll be interested to see what other people come back with as I'm in a similar situation myself (wanting to invest my cash within a SIPP into something low risk and that's liquid).
Regards
Money Biz