Hi Steve,
I know this is an old post now but as no-one else has answered I'll try to provide some pointers.
Architecture with a 300mm lens is a good challenge, it allows you to zero in on isolated details in the architecture and it helps to simplify your composition and eliminate distracting elements.
These are all good attempts and it's clear what you want the viewer to focus on, but whenever you're using a zoom lens you need to be aware of the finer details in your composition - the lines / angles / perspective all need to work together to support your subject.
Your first image could benefit from shifting much further to the right, I can see you're framing the spire within the arch, but it's not quite coming together in the frame. My guess is you couldn't get to a location which would allow you to shoot square-on to the arch? If you could, this would create a nice symmetry in your image. As it stands here, the composition is weighted too heavily to the left.
Your second image is a bit the same, if you could have moved to your left more you would have been able to square up those lines evenly on both sides to create symmetry and balance within the frame. You don't always need to do this, but if you're shooting on an angle you still need to make sure that the lines in the frame are working towards a compositional goal - cutting off the left side of the sign feels unintentional, but perhaps not?
Your last image of the gargoyle is a tricky one. The tones in your monochrome image are blending together and it's concealing your subject. In one sense you've got some good leading lines pointing to the subject, but in this image it's not working. I'm not sure what you could do here to improve on the shot - re-positioning yourself so that the head of the gargoyle is framed onto a lighter background within its own space may work, or perhaps it worked better in colour if the head was a different colour to the background?
I think you've got the right ideas, you're pulling out interesting subjects and making attempts to frame them within your image - a few tweaks to your positioning when taking the shot should bring it all together.