Hi @Bristolbadger and all!
I think I may have come across a solution...
I noticed that if I logged out from the All4 Roku app I could browse the HOME tab ok. It was just when I was logged in that the HOME tab caused a return to the Roku home page if you paused on it and usually it has some entries of shows that I am currently "Watching...". If I tried to get to my My4 tab above this it also restarted, unless I could get off "Watching..." entry and down to "My List" before it displayed any thumbnails.
So I logged onto www.channel4.com on a web browser and deleted all the "Watching..." entries. When I restarted Roku and the All4 app and logged in again, I can now sit on the HOME tab without it crashing.
I've only watched one show so far and hasn't crashed yet, but whether it will start crashing again once my "Watching..." list fills up is another matter. However I did notice in the web browser that some of the shows that I had watched were in a separate list marked as "no longer available" I wonder if it was trying to display something that no longer exists.
Hope this helps anyone else get functionality back, shout out if it works for you too!