Hi Luke,
I'm in a similar position looking for a chain guard for my 54 frame. The two I have with the box of bits don't fit and not sure what they are off. I haven't looked in earnest, but please let me know if you find anyone selling new ones in the UK. If your selling the old one drop me a PM as I may be interested, as its cost conscious for me to buy a new one.
Kind Regards
Dave