On the whole, I think it looks great, if a little big for a dubbel - be sure to mash really low to get it dry enough. Personally, I'd dump the chocolate, IMO it will do nothing for the beer, the roast flavour is not part of the style. If you want to get the colour a little darker, you could use some carafa II, but personally I wouldn't bother. You should also really boil for 90minutes to reduce the DMS produced from all that pilsener malt.
Also, you might find that a few percent wheat malt will do wonders for your head retention, and if you keep it low enough it won't make the beer hazy.
As for hops, traditionally these beers have almost zero hop aroma, so you could just go with one hop addition at 90 minutes - I'd use all hallertau or tettnang, although magnum is probably just fine too. 26 IBUs does seem a little bit low for such a big beer though, I usually shoot for a BU:GU around .375
What candi sugar are you using? Don't use the rocks, even the dark ones will not give you what you're looking for, you really need syrup. Worse, in my experience, making your own will not yield the best results, contrary to what some people may tell you. You really need the real stuff from Belgium. You can get it from Brouwland
, they will ship to the UK and it's not too expensive iirc. For such a big beer you could add it to the fermenter as primary fermentation slows down, this will ensure you get maximum attenuation as the yeast will concentrate on the malt sugars first rather than the simple sugars in the candi syrup, which could possibly result in some residual fermentables left in the beer.
I think that WLP500 is the chimay strain, you'll probably want to ferment fairly cool, at least at first anyway. That strain makes major banana esters when fermented warm. This beer would probably benefit from a month or so of lagering if you have that capability.