The bottom half of the window has come loose, the glue must have given up, has anybody had this issue before, can I just stick it again with silicone

The glass should be bonded to the opening with the correct adhesive not silicon, as a temporary measure if its letting in water then run some silicon over it but you need to get it properly re-bonded

Thanks, Where can I get the bonding from?

You could get it from VW dealer

16) DA 004600A2 2k-window adhesive set 400ML /

(16) DH 009100 1k-window adhesive 300ML /

(17) D 181801A1 activator for precoated windows 10ML /

(17) D 00920002 primer for glass and paint 10ML /

or try autoglass or similar screen fitting places

Take it to an independent glass fitter or even Autoglass. You'll be surprised at how reasonable their quotes can be. They'll do a good warrantied job in half the time. Water damage is an absolute pain to deal with.

Just pick up the phone and get some quotes!

PoloD.