Work on the assumption that the agency work has dried up for good.
Sign on (which you're perfectly entitled to do) and apply for Local Housing Allowance (which has replaced Housing Benefit for new applicants) as well as Council Tax Benefit.
If, in future, you get less than 16 hours work in a week you'll have to declare it (and suffer the subsequent loss of benefits) but you won't need to 'sign off'.
If you get more than 16 hours work in a week you'll have to sign off but you can immediately sign on again (under the 'rapid reclaim' system) on the day after you finish working. (It's a pain in the @rse because it means attending a fresh interview every time you do it, but at least it keeps the money coming in).
Theoretically you need to end your claims for LHA and CTB, and then reclaim them, every time you get more than 16 hours work in a week. However I've found that simply writing a letter to the council's benefits section, giving details of the temporary work and the pay you got for it, saves the trouble of having to fill in lengthy forms every time.
Chris