Thanks for the further updates.
I would describe myself as a novice marathon runner (never done one before) but am a competent runner (doing 1,000+ miles a year) and with good PBs (sub 20 5k, sub 40 10k, sub 90 1/2).
Based on advice here I went to a local (very friendly) running shop and spent 30/40 minutes testing lots of different styles and brands.
The end result was the rocker/foot plate ones didn’t feel right - not enough of a connection to the ground for me (might try a pair at some point but not at the start of the 20 weeks training plan).
I tried a few minimal cushioning ones (which is what I have been running in) and then mid cushioning and finally the ‘big cushion’ styles.
In the end I have selected a mid cushioning style, and went for ON as they are a brand and fit I have run in a number of years and found comfortable. They are also really improving their sustainability so also felt the best environmental choice as well.
Wish me luck!