Thanks for the reply.
I have seen how to set a featured image in the header media and I have cliked to make this sitewide as my banners have mostly dissappeared. I havent changed any CSS code.

It is a bit weird as one page sill has its Banner image which is its featured image: https://www.davetoddpt.co.uk/pricing/

All my other pages just display my Header image now as the banner despite having their own Featured images set: https://www.davetoddpt.co.uk/