I've received a reply from Osprey Publishing regarding this website. See below
I have talked with our Games manager, he just wants me to make it extra clear that the issue was that you were asking for a donation in exchange for additional resources; if everything is for free, with a donation being a 100% voluntary aspect, we are happy for you to re-instate the donation button.
Let me know if you have any doubt, I'll be happy to reply.
I think that's a pretty reasonable outcome and its nice to see a company taking such a stance (yeah, we are looking at you Games Workshop). I now have to decide if I want to release the extra functionality or not to everyone. I will consult with the people who have supported development of this site with donations as I don't want to devalue those contributions without consideration.
I will try to do this over the Easter weekend and deal with the outcome the following week.