What kind of spider has colonised my timber framework?
You've encountered a very common spider, found all over holarctic.
Common names: European garden spider, diadem spider or cross spider
Scientific name: Araneus diadematus
link to wiki:
https://en.wikipedia.org/wiki/Araneus_diadematus 

They can bite, but are hard to provoke.
If bitten it will feel similar to a wasp sting.
So nothing to worry about, unless you are allergic.

picture from underneath:

source: http://c8.alamy.com/comp/H45CTY/spinning-network-garden-spider-araneus-diadematus-bottom-side-animal-H45CTY.jpg

picture from above:

source: enter link description here