Gaining

I’ve been taking 300-450ml of double cream (1,400-2,100 calories) almost each day for 1 month but I weighed myself and have around the same weight as before I started somehow.
Am I doing something wrong?

A lot of people think that if you consume a lot of heavy cream, you'll gain weight. This isn't always the case.

Heavy cream is used for weight loss and weight gain. Why? Because it's so rich that it can kill your appetite. So, if you are barely eating anything in addition to the cream your drinking, it's not enough to gain weight.

Another thing to keep in mind that your body cannot use every calorie you consume. If you consume too much at once, a good chunk of those calories will be flushed down the toilet. Plus, since heavy cream is high in fat, it can have a laxative effect as well.

Lastly, there's your metabolism and activity level. In order to gain weight, you have to burn off fewer calories than your body can store. If you are fairly active and/or have a high metabolism, your body will resist the pounds.

So I have the cream after breakfast and dinner (just before I go to bed) and sometimes around 5pm if I get the chance. but I thought I would have gained something as nothing else has changed I just added cream to my usual routine.

Well, what is your usual routine?

So it’s 150ml in the morning with breakfast. 150ml when I come back around 5-6. Have dinner an hour or 2 later and then around midnight have the last 150ml. I do have snacks in between too.

About how many calories do you eat a day? How many do you burn?

So from a rough estimate in food around 2000 calories and with cream should be +1400-2000 on top. I should only be burning around 2000 do don't do much walking and don't do any proper exercise.

And what is your basal metabolic rate? What kinds of foods do you normally eat? What do you normally drink?

There are many factors that need to be taken into account, like Munchies said. Also, weight gain is a slow process. Your body can only digest so much so fast. To much cream all at once will make for an unhappy digestive system and a waste of calories. With heavy/double cream start off slow. See how your system takes it. Drink it throughout the day. I usually have it after eating. As you mentioned you need the base 2000-2500 calories everyday just to live. Doing some exercise would actually be good and helpful. Even an hour of exercise doesn't burn much. It's better for your overall health and won't affect your gaining much. Unless you do a lot of very rigorous workouts.
Doing all that, yes, heavy cream can add a ton of calories to your daily intake and result in surprising gains. It's singlehandedly what has helped me in all my gains. But the biggest thing to remember is it takes time. Even of your gaining a couple pounds a week it can take a bit to notice it. Best of luck!

From what I've read on this site, the body can only digest so much in a short period of time, the figure I've read is about 1000kcal in 1-2 hours.

Given that 100ml of double cream is about 467kcal that would mean 200ml is 934 kcal.

2-4 weeks is also typically what's stated to see some affects, although in my case it was days after starting.

Be careful as well how you take it, I found double cream without something to 'thicken it' (ice cream in my case) gave me stomach problems.

Given your dietary habit, it might be worth totaling up the typical kcal of what you eat to see what you're intake is.