Hello! I am also in for Abingdon this year, it will be my fifth marathon but first at Abingdon.
I also ran Brighton this year Robert, hoping an October marathon will be cooler! Only just started proper marathon training due to being on holiday, so hoping a 13 week build up will be enough, we'll soon see.
I also now use High 5 Isogels without any problems but it did take a lot of trial and error to find what worked for me. I found that by using them on long runs my system learnt to tolerate them better.