In reply to astley007:
Try counting calories, it will always help as it's difficult to know if you've eaten enough. I recommend using MyFitnessPal Android/iOS app to log food. Aim for 500 calories above your base rate to gain 500g of weight per week. Note that a lot of that will just be fat.
With that said:
Sandwiches with lots of mayonnaise and cheese in them. (put lots of interesting fillings in your sandwiches.)
Cook more than you need. If you're boiling potatoes, make double, and slice some into a sandwich for lunch, slice some into a frying pan with lots of butter for breakfast.
When making pasta, go heavy on the olive oil. Drizzle extra on "for presentation"
Switch to full fat milk as opposed to semi-skimmed. (every little helps). With that in mind, try cream in your coffee.
Macaroni cheese (or pasta with roux sauce). Make Lots of roux. Add Bacon. Bake for extra crunchiness.
Fancy a slice of toast and jam? Why not fry the bread with lots of butter, then put the jam on?
All that said, If you want to bulk "Healthily", you should take care of your macros, and get a good proportion of fat:protein:carbohydrate, that way it won't all be fat.