The term STC (Supplementary Type Certificate) may possibly not be valid in a UK CAA context for what you are talking about.
An STC is generally awarded to a piece of series kit that is supplied to be fitted straight to an aircraft. The STC if it exists will be held by the equipment supplier and they'll supply you with all the paperwork and guidance you need for a LAME to install and sign it off.
More likely these items have been installed as minor mods. As A&C says, if this is approved for all identical aircraft then the same applies. If it wasn't you need to make a direct application to your CAA regional office. The process is moderately painless, likely to cost you an admin fee of around £70 or so, but it would help enormously if you can get hold of the original application from whoever made it and copy that.
If you want chapter and verse, thing to do is phone the CAA on 01293-576161, ask for "Aircraft Certification", then when you get through to them ask to speak to the "Design Liaison Surveyor" for Piper aircraft. They should have the list of what's approved for all identical.
If CAA make life difficult for you give me a shout, they're usually happy to accept my signature on such things and I'd happily swap whatever time and effort was needed for a bit of Cub time.
G