Pete, I think there is a CB place on the industrial estate between the big duel carage way and Billing Aquadrome main gate, might be worth a look if your that way.
I've got the truck mirror mount on mine (genuine T3 mirrors with the longer flat bit at the top), avalable from any HGV radio dealer. SWR is very good from it. No no cutting or drilling of van needed as I routed the wire through the door under the lower mirror mount through the middle hole (I trimmed the gasget slightly so there was more space for the wire between the body/mirror). The areal tip is over 3.5m above ground so I sometimes bend it round and tuck it in the window if I'm driving indoors - saves it twanging off everything.
http://www.kcb.co.uk/shop2/index1.html
doesn't seem to link right in so go to (mounts&bases>vehicle>body) on sidemenu.
I also have the DV mount (second down) mounted through the standard areal hole in the front pannel, covers the oval well. That ones got the smaller length springy areal on it which goes up to about roof hight. Aparently they are rubish areals for CB, the longer one is what you want. I plan to use it for FM which the short one is perfect for. Might work with a longer wip on there but it might swing into the windscreen a bit if you try driving through one of those thicker hedges. Still havn't wired it up though as I can't solder a DV plug without wrecking it! I think I'll buy a lead with a DV plug pre atached and cut the other end down...one day! Can't see that mount performing better than the mirror mount as it'll be lower but it probubly won't be much worse and you might prefer the location.
Another thought you could try googleing "ERF mount" as they had fibreglass cabs so the bases needed artifical ground plates to make them perform properly. Are more expensive though and I think many still use the mirror frame.