Clive
I did the Frankfurt flight in 2014 having left Brisbane the previous day. Also had around a 22 hour layover. Booked the Novotel Clarke Quay (if an Accor member you can get rates of around $180 in Accor sales). Was able to enjoy a great dinner that night and long walk with good local breakfast the next morning. Did not leave the Novotel until around 11.30 the next morning which left around 2 hours in the lounge.
Another Accor option is the Ibis Bencoolen which is around $100 annd very well situated.
We had a night in T2 Transit hotel in July this year on the way to Sri Lanka. Booked for 9 hours and cost was around $185. Romm was adequate though an internal room only.
If transit time is 22 hours that is pleanty of time to enjoy Singapore!