I followed the Crab Mac 'n' Cheese recipe to the letter (with my new digital scales I got for Christmas) but with a different-shaped pasta rather than shell pasta. My sauce did not thicken to the "Velveeta" consistency mentioned. Could it be the supermarket crab packs to blame?
Nigella's Crab Mac 'n' Cheese (from COOK, EAT, REPEAT) has a cheese sauce made with 250ml milk, 15g plain (all-purpose) flour and 30g butter. This amount of flour and butter should be enough to give a sauce with a thick-ish coating consistency. The Velveeta reference is to the ready-made sauce that comes in a squeezy bottle or can be made up from a package.
Usually the cheese sauce from a mac and cheese will thicken slightly more when it is added to the pasta, as the starches from the pasta mix with the sauce and also the pasta itself (regardless of shape) will absorb a bit of extra liquid from the sauce. If the sauce is on the thin side it could be that the crab was packed with some liquid and if this liquid is added with the crab then it would thin the sauce slightly. Otherwise the sauce may not have been cooked quite long enough to thicken and may have needed a few more minutes on the stove before the cheese was added. If this is the case then you may still have a faint taste of flour. As it is a roux-thickened sauce you can bring the sauce (without cheese or crab) up to boiling point if you are not sure, but just be careful to whisk or stir continually while you do this so that you don't end up with a lumpy sauce. Finally, if you cook the flour and butter roux for too long then its thickening ability becomes less, so remove the pan from the heat as soon as the ingredients form a paste.