ESA and IS/Incap conundrum

Client was paid reduced rate income support whilst appealing PCA decision of April 2010.  His appeal was heard and dismissed in Feb.2011, after which he made a claim for ESA.  Client managed to get the PCA decision set aside, and appeal was allowed in Sept 2011.

Client was paid arrears of income support from April 2010 (date of PCA cut-off) to Feb 2011, but DWP won’t pay IS from Feb 2011 (date when ESA awarded).  Client is still getting ESA, and is receiving it at the assessment phase rate, pending an LCW appeal, being heard on 12/1/12.

Am I right in thinking that the IS claim should be reinstated (and the ESA claim terminated) and arrears of IS to also be paid for the period from Feb 2011 onwards, in accordance with DWP guidance DMG 16/10, para. 20, DMG 33/10 para. 22 and Reg. 5 (2), Case 2, Payment on Account Regs. 

Or have I overlooked something.  A second opinion would be much appreciated.