This has almost definitely been asked before - but couldn’t see that it had recently come up.
I did IM Austria last year (my first IM) and am doing Switzerland this year (sucker!). Last year I got my nutrition wrong. Stomach cramped up and wound up on run (walk) with what felt like a brick where my stomach had been. I drank what I thought was my quota on the ride (SIS GO at 6% solution) and used PowerGels and a PowerBar - this I had done in training. I can’t remember but think I was aiming at a gel every half hour.
Did I consume too much sugar? I am a shade over 12 stone. How many calories should I be aiming to consume on the ride? Being a Brit I’m not used to heat so need to be careful in Switzerland to handle that requirement too. I have found that drinking higher cal. count drinks (SIS PSP22) doesn’t permit me to remain as hydrated.
I presume you are in the UK from your message. High Five make a great Isotonic mix to drink at 6-8% this will deal with your Carbs and salt if you drink 1000ml per hour. Any more carbs from gels or drink and your body can’t digest it. You need to work out what you will do when your bottles run out. Options are the course drink/ Water and Gels or bars / dumping drink mix in a aero bottle. Enervit (recommended by Tom D) make a neat looking foil drink pack with tube that can be squeezed into a aero bottle or sipped with water separately.