Replacement stained glass in a door.

[leew2]

Hi, have a door which is about 100 years old with a stained glass window in it. The window is made up of about 100 pieces of glass held in place by H-shaped lead strips. One of the small pieces has cracked.
If I was to replace this piece of glass, which measures about 3 inch square, do I have to use tempered glass? Or are very small panes except?
Many thanks.

 

[chapeau]

It's all in the approved document N

http://www.planningportal.gov.uk/uploads/br/BR_PDF_ADN_1998.pdf

1) It's a repair so not subject to building regs anyway.
2) If it was, it's a small pane so 4mm glass should be OK. (See 1.5, 1.6)