Is there, or is there not a list of mandatory requirements for a first aid kit in the workplace?

The quick answer at the time of writing in that there isn't a mandatory list but there is guidance as to what should be included.

There is a British Standard for first aid kits though (BS8599). It's not compulsory that you stock these kits, but it is compulsory that you carry out a first aid needs assessment which covers the equipment you will provide.

More information direct for the HSE can be accessed here.