It is probably just water. I promise! 0.4 lb is nothing.
Given that you are 5'9" and weigh 19 stone 12, you would need to eat 2350 to 2550 to MAINTAIN your current weight (I didn't know your gender or age, so I used my own for the calculator: female and 32). That's with very light activity--just above being bedridden. To gain weight, you'd need to increase that to 2850 to 3050 per day. Obviously, your calorie needs might be higher or lower than this, but it gives you a general idea. Unless you carried on the rest of the week having 3000 calories a day, I wouldn't worry about one slip. You'll drive yourself mad if you do. Just treat each day as a clean slate and do the best you can.
One bad day will not set you back that far. Best to put it behind you, stop beating yourself up, and move on. There will absolutely be setbacks along the way. No one has a perfect journey. But you don't need to treat yourself like a failure after one slip. Be kinder to yourself than that. And yes, as chezz says, stop weighing every day! Those little water weight fluctuations can be so disheartening. Once a week is the maximum, I'd say. Once a month is probably better, but I don't have that kind of willpower.
If you want to avoid having the horrible feelings that come along with indulging (they're hard to get rid of!), what I do at social occasions is plan it out as much as possible beforehand. Like, on my husband's birthday. I knew we'd be having pizza and cake, so I calculated how much I'd be able to eat and still stay within my plan. If I go over, I try to keep within the calories to maintain my current weight. That way I feel like I'm at least standing still rather than going backwards.
