I’m finding my 3 1/2 month old is uncomfortable to lay down particularly after feeds. I have him up right on me to burp and he will fall asleep comfortably in this position or when walking in the pram which has a slight tilt. When I put him down flat he squirms and appears uncomfortable. I am aware nothing can go in the cot so is it ok to prop the cot up slightly at one end from the outside? Or something to put under the mattress to provide a slight incline?

Red Nose recommends that baby is placed for sleep on their back & flat (including those with reflux)
Red Nose has comprehensive information on web site, some of which is quoted here.
Babies with GOR (Reflux) should be placed to sleep on their back from birth on a firm, flat mattress that is not elevated.
If a baby is in an elevated cot, further hazards may be introduced into the sleeping environment. When elevated, babies are more likely to slip down the cot and become completely covered by bedding, or if a pillow is used to elevate the baby pillows become a suffocation hazard
Sleeping baby on the back provides airway protection 6. The back sleeping position is safer for babies with GOR as babies can protect their airways when placed on the back compared with babies placed to sleep on the tummy or side
https://rednose.org.au/article/sleeping-position-for-babies-with-gastro-oesophageal-reflux-gor

There is a risk to baby of being placed to sleep in a position that may facilitate baby’s neck being flexed to the “chin on chest” position, especially when sleeping in that position for extended periods & unobserved.
There have been accidents associated with bouncers, rockers and hammocks such as falls and entrapment, but the biggest hazard is the risk of a fatal sleeping accident.
When a baby is put to sleep in one of these devices, their head can roll forward, blocking their airway.
https://rednose.org.au/article/baby-products-you-dont-need