I have decided to go with the original petrol tank after finding out it is nearly new , I'm going to cap off the original filler tube and what is I assume a vent pipe next to the filler tube , any suggestions for a new vent system ? cheers Jon   

There may be a breather pipe on the left of the tank you can run a hose and pupe from to stop spills and let air in, the 14mm ish vent near the filler is to allow air out into the filler pipe when refilling to reduce blow backs.
A bent pipe needs to go the width and length of the tank then higher before venting under the tank or fit a breather/spill valve on a short pipe.

I've always just drilled a tiny hole in the filler cap to vent things. Nice and simple.

Any way as long as you have a vent. If you add a one way/anti roll spill valve go the extra mile and pay for a decent one... not ebay chinese as they don`t work after a short while. 

I've always just drilled a tiny hole in the filler cap to vent things. Nice and simple.

Same as David 2mm hole to the side turn

I've always just drilled a tiny hole in the filler cap to vent things. Nice and simple.

Don't you get a smell of petrol from the small hole ?

I've always just drilled a tiny hole in the filler cap to vent things. Nice and simple.

Don't you get a smell of petrol from the small hole ?

No, you only need a 1mm hole and you've got a windscreen and a lot of air blowing round between yourself and the filler.

What do you guys use for a filler neck/cap ? the ones I've seen on the bay of thieves seem a bit on the large size at 80mm , what is the ideal size to go for ? cheers Jon

Log onto carbuilder solutions. Navigate to "catalogue"   (top of home page). Order copy...    This is a free. very comprehensive catalogue.
Others such as Holden, Merlin etc, etc. By doing this you will see what is available and at what cost....  Tip here.... if the choice you look at is "chrome" or stainless go for stainless. I have a top quality, supposedly, chrome flip top filler cap. Despite oiling it regularly the surface is now showing pin pricking... This after a mere 5 years.

  https://www.carbuilder.com/uk/car-builder-merchandise

I've used these in the past and they work really well. The cap flips open hiding a locking fuel cap.

You'll need a filler neck and grommet off an old BL car, think Midget, or MGB to fit it.

https://www.ebay.co.uk/itm/272277131690?mkcid=16&mkevt=1&mkrid=711-127632-2357-0&ssspo=XNUIQibMRp-&sssrc=4429486&ssuid=QRti4rzaTW6&var=&widget_ver=artemis&media=COPY

  Thanks everyone , gotta love this forum !

We have a vented locking cap with a 50 mm pipe from CBS

I've noticed nearly everyone goes for the classic centre mounted filler position , does anyone have problems leaning so far over the buggy to fill it up , any disadvantage in offsetting the filler to the drivers side just for ease of access ?   

Centre fill stops surge leaks on corners, acceleration and braking depending where your filler is located. Quicker to fill up and you can use a stick to measure contents level.

I'm guessing that the standard VW sender unit is next to useless due to the angle of the tank ?