Here's what I would do for best results. It's kind of expensive, though.

I'd skip the Kodak IT8 target which has a minimal number of color patches and purchase a Hutchcolor (HTC) target. They come in two films: Velvia and Kodak E6. I would confer with HTC to determine which has color characteristics most like Provia. My guess would be the Kodak E6, 6x9 target which they sell at about $5C each. But, these targets include 540 color patches. Along with the target, you get a data file that can be used to create the profile.

Under the exact same lighting that you use for camera scanning your transparencies, photograph the target using your Leica camera, and convert your target image to a tiff file. There should be zero modification of this image file. No corrections for color temperature, etc. None.

Work with a service provider to render this target Tiff image as a profile. They would need the datafile that comes with the target. Being in the Pacific Northwest, I would probably use Chromix in Seattle. Not sure about what's available in the UK, but you might be able to find a color management consultant who already has the HTC target that you would need. Hard to say if this is viable. Even if viable, they might have a faded target that's fallen out of step with their datafile.

In scanning a one of your transparencies, process the image file in exactly the same way that you processed the target tiff file, and then assign in Photoshop the profile that you've developed. This should render an image file that's closest to the camera scanned transparency. Though, some image manipulation will likely still be needed.