Neill
Cahoot provide a specific code when they issue their final resolution letter. If you complain to Cahoot, they should write to you within a few days and provide you with a letter declining your complaint and referring you either to complain to Abbey or talk to Abbey about rearranging your loan. Their initial complaint investigation is quite prompt.
Cahoot provided me with an address for Abbey in relation to 'stage 2 complaints' (escalated complaints), which was incorrect i.e. My two subsequent letters were ignored and not responded to at all.
I ended up writing to the Abbey complaints address on the Abbey website referring to my previous Cahoot complaint. It took almost 6 months from start to finish for Abbey to provide a 'Stage 2' final resolution letter.
If you need to transfer more of your flex loan across, it might be worth you obtaining a code, although given their responses so far I would imagine the rate offered would only be a small discount.
Hope that helps. I would complain irrespective of the outcome. It will cost Cahoot and Abbey more money, just as their rate increases have cost consumers more.