Just got off the line to Plusnet and they confirmed that i need a MAC from my old supplier...
If you are fully LLU'd with sky then the Mac code system is not applicable, plusnet would need to do a return to donor order basically, to get your line re connected or whatever needs doing before broadband services can be ordered on this line, and as result of the line being disconnected from sky's llu any active broadband will not be present on the line after plusnet have had it transfered to them
But if you are SMPF Broadband only is on sky's LLU then yes the mac code process would apply and sky are obligated to supply you one upon your request
Possibly the easiest way of you finding out if your line is full or partial llu is to input your telephone number into the bt wholesale adsl checker, if Full llu's it will not give any speed estimates for your line,
failing that run a tracert and if the 1st hop after the router is an Easynet.net switch, then you most probably will be only SMPF partial LLU so would there fore require a Mac code