Going through G-INFO brings up this page:
GINFO Registration History | Aircraft Register | Safety Regulation
which subsequently leads to this:
http://www.caa.co.uk/docs/HistoricalMaterial/G-AZOR.pdf
So, G-WAAN is Ser No 20 built in 71/72. The German Army didn't order 105s until 1977 so the BBC were incorrect about it's history.