This is not so much a gap in the market, but a symptom of the market.
All short term cash investments are paying derisory returns. You need to accept it until the banks again start competing to attract capital from investors. Currently the banks can obtain capital for a lower cost from the government and they are under no pressure to raise savings rates.
Staying in cash is, in most cases, not a bad thing given that inflation is so low, and likely to stay low for a while. As soon as you exit cash then you take on higher risk and it for you to decide (with a financial advisor?) what risk you are willing to take.
I for one like the peer-to-peer lending models for the premium returns that are available. Some of these platforms will take SIPP cash but it ties the money up for a period.
I hope the above helps - please accept that I not a financial advisor and will not be held responsible for anything I have said. However, if you use this information to make money then I would like my 25% share :-)
regards
Geoff