As others have said the outer belt is the power steering belt, the Haynes book is hopeless and doesn't seem to realise that these cars came with PAS.
I found this guide helpfull when changing the water pump which involves removing both belts as it actually identifies the adjustmnet bolts:
http://www.volkstorque.co.uk/vt/mech-tech-questions-answers/62134-mk2-golf-thermostat-replacement.htmlThe third photo shows the hidden bolt!
I assume that you have the KR engine, there are two different sizes of belt for PAS depending on the chassis number, I found out by buying the wrong one first based on the guide in Halfords. According to VAGCAT the options are:
v-belt / F >> 1G-MW240 000* / F >> 1G-MB075 000*
9,5X730MM / KR,PL
(7) 027903137
v-belt / F 1G-MW240 001>>* / F 1G-MB075 001>>*
9,5X630MM / KR,PL
(7) 026145271