where do you buy your seed?
I don't grow large onions anymore but when I did I used to buy them privately from a top grower.
As that was nearly twenty years ago I have lost touch with him!
I think Medwyn's seed as linked in that article might be as good as any.
I have noticed that at the shows his onions seem to be that bit better looking than Robinsons, and as I mentioned before that is not to say Robinsons are not good.
If I remember when I go to Harrogate show on the 18th of this month I will look at what the top growers are growing now.
Places like this is a good starting point when setting out on the 'Big veg ' trail at least you can get the chance to pick up the best seed for the job.
But getting the right seed is only part of the task of getting big veg, as they can be quite labour intensive if you want the success.
I was always taught that to succeed "the results you get are commensurate with the effort YOU put into it"
So do not think because you have the right seed that you are guaranteed success!
I have always tried to stick to this regime. In fact I still pretty much apply the same attitudes today as old habits die hard.
As an aside I always thought that winning the under half a pound onion was a better class to win simply because more people entered it and weight was critical, plus the also ran onions you picked to get your three or five exhibits, tasted better than the giant ones. So with these I got the best of two worlds.
So the best of luck with your venture.
BTW sowing on Boxing Day is an old wives tale, the top growers onions might be the size of Spring onions by then.
In the end it all comes down to the facilities you have!