Leaking from either the cap or gasket under cover

I have removed the cover and checked the gasket/head seal - it's not cracked.

However, it recently started spewing out steam to the point where the black cover appears to have almost melted. After reading some of the answers here, I'm wondering if there's plenty of calcification within the boiler. Would you know where I could buy the head seal - I haven't been able to source it. Any advice would be appreciated. Thanks!