It will be an MSc that you'll be looking to undertake as an MEng is an undergraduate degree (ie it's a 4/5 year course including Hons degree as oppose to a 1 year Masters).
Accredited programmes here - http://www.theiet.org/academic...opsis-accred-list.cfm
If your going to be studying whilst working then it'll have to be at a uni that is reasonably close to you - where are you based? If you're planning on studying at a uni that's not close then you'll have to look at Distance Learning MSc.
Would defo recommend IEng on route to CEng; that's what I have done. It will give you experience in pulling everything together and also an insight to the application/interview process. That way, when you come to preparing for CEng, it's not completely new. Plus, there may be a cash benefit from your work for achieving professional registration.
In terms of help from the IET - as the others have mentioned, applying for a mentor would be a good start. Although, I'm not sure they can advise on Masters courses.
JBB IEng MIET