I know this is probably a silly question and I'm guessing it will vary from person to person, but what would be an average training period.
It will also sound silly as I can't even run 5k yet, but I'd really like to try do a marathon in the future to raise money for the MNDA even if it is in a fair few years time!
Any advice/tips would be hugely appreciated! Many thanks in advance. 
I suppose it depends how seriously you want to take it.
I would have thought this time next year would be feasible and not unreasonable if you are serious.
Next year should certainly be achievable, especially if you are content with "just" getting round, rather than aiming for a time. If you Google Marathon Training Plans you will find plenty of options, including a "get you round" plan. Or look for Jeff Galloway's website, which has plans with plenty of walk breaks.
The C25K is a good place to start though
I've heard a year is the most sensible amount of time to train for a marathon starting from scratch. You should just get going with C25K then build up your distance and see how it goes.
There was a link posted recently here for an inspirational video of a chap who went from being obese and unfit to marathon 9 months later. Lots of non-running training by the look of it.
How long did it take you to actually run 5k?
How to do a 6+hour marathon
Guess how long I'll take to run the Great North Run
training for a half-marathon
