@Kingfisher; welcome to the forum. Lots of factors can influence fuel consumption and bear in mind the official fuel consumption figures aren’t achievable by most drivers.
If your driving is mainly short journeys where the engine doesn’t have an opportunity to warm up fully, your fuel consumption won’t be great. Similarly in if a high proportion of your journeys is in built up areas at peak times, there’s a good chance you’ll be doing lots of stop start driving in lower gears - again not good for fuel economy. Are your tyres at the correct pressure? - under inflated tyres will have an adverse effect on fuel consumption. Also, I find my fuel consumption during the colder winter months tends to be worse than it is during the warmer months.
How are you calculating your mpg? Over what distance is the 35/36 mpg figure based on? If your car has an mpg display and your 35/36 mpg is calculated by the car, bear in mind that the figures calculated by the car might not be accurate. IMO, it’s better to calculate your mpg manually over a number of full tanks of petrol, by recording the mileage at each fill up, and divide the mileage driven on each tankful by the number of gallons of petrol used.