To my surprise I found that there were four screws strategically placed for mounting roof bars and roof racks on the vehicle. Removing one of the screws proved that a secure fixing could be made to these mounting holes and so I went in and started to check out ebay for antenna mounts.
I found a suitable antenna mount from Truck King Radio (who are now sadly defunct) on that well known auction site and trusting in my judgement (not usually a good idea but for once...) The order was duly placed and the goods arrived two days later. when they arrived I looked at the modifications that would be needed. aturally the bracket was the wrong shape to just go straight on to the roof, the plates have ridges in them for mounting on roof bars and so a short period in the garage with a blowtorch and a lump hammer straightened out one plate. An extra hole to clear an M8 threaded machined screw was drilled into the lower part of the plate and a small piece of rubber cut out from a larger sheet to go between the bracket and vehicle roof to prevent damage to the paintwork.
A coat of silver Hammerite paintwhich matches the car (and just happened to be lying around in the garage) was then applied to the bracket which was done for two reasons 1) to make it look pretty and 2) to protect from corrosion.
Also from the same place I picked up a matching SO239 cable assembly. This is a single hole mount with a length of coax connected to it and a PL 259 on the end. The quality is not over marvelous, the crimped connection at the SO239 end will allow water ingress, particularly in winter (and UK summers). Some clear sealant soon put a stop to that problem. The SO239 mount was mounted on the bracket along with a short length of braided wire to go under the penny washer for the mounting screw ensuring a good earthing connection to the car's bodywork.
The original screws provided by VW are made from a softish material and are not very long. Exploring the Screwfix website I tracked down some hex socket set screws which were the correct length and made from stainless steel. OK so I don't really need a picture but they are rather nice looking.
The last part of this article shows the finished result.