Thanks! That's interesting because it ties in with the Norwood burial record, which gave his residence as Gravesend. 170 Strand was the office of The Observer! Despite everyone giving it as their home address, no one lives in it on the 1841 and 1851 census (unless they were busy, it being a Sunday night and them running a Sunday paper!), and when someone does in 1861, it's not the Clements.
I think I've now found his death certificate at least - there's a Mar qtr 1859 Wm Clement death in N. Aylesford, which seems to be the right area.
I wondered if he died intestate? His half-brother George died in 1861, aged 47, and he did have a will (he died in the November and probate was granted on 2 Jan 1862). I've sent off for that so maybe that'll reveal something.