Hi Bill, I made a search myself with the same results as you, so no answer other than the website is not brilliant. I found it a little troublesome to use.
I can only say calling them did the trick, I just said I wanted bridge seals for Girling calipers and they asked what the size was due to there being 2 sizes. I got them the next day with the part number shown on the bag and invoice. I didn't make it up.
Maybe its something to do with the scare tactic of telling you that you must not split calipers.
As for getting calipers rebuilt for about the same as buying the parts as Johnetype suggested, I very much doubt that could be achieved, if it was possible they must rebuild them for nothing.
It cost me 70 quid to rebuild my rear calipers, including plating, 4 new pistons (standard not st/st) and new seals and I had the pleasure of doing it myself which helped relieve the lockdown boredom.
