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The rear lens appears to be very tightly fitted. I assume the rear lens should easily come off, and then i can remove the retaining ring?

Any advice welcome.
Thanks
Bip

Hello from France !
In most wide-angle view camera lenses you need to unscrew the rear lens group before attemping to unscrew the retaining ring.
Normally the rear group should come out simply by unscrewing by hand.
However in case the lens mounting screw would be blocked for some reason, you could use a dedicated tool, a rubber strap wrench / grip wrench.
It looks like this :
http://fortunel.manufacturer.globalsour ... roduct.htm

But you can probably find in a hardware store find a similar tool used to unscrew the "twist-off" lid of, say, jam jars.

Once the rear lens group has been unscrewed, store it safely so there is no risk for it to roll off the table and no risk for the lens elements to be scratched

Now comes the second part of the job: unscrewing the shutter retaining ring. Of course a single screwdriver is inoperant and can even be dangerous.
Usually a simple dedicated spanner wrench made of a steel plate with proper cuts for different ring sizes allows to do the job.
Linos-Rodenstock or Toyo offer such a tool for about £15.
This is how this simple tool looks like :
http://camerashop.in/images/new%20tool% ... eb.psd.jpg
http://www.adorama.com/images/Product/TY180625.jpg

Both tools, the rubber strap wrench and the spanner plate wrench are affordable and very useful tools that any LF'er should have in his assortment of small accessories !

many thanks for your reply Emmannuel.
the rubber strap will be useful to remember.

i did remove the rear lens, i was being too cautious because it is a borrowed lens. i think because it has not been removed for a long time it was just the initial extra force required.

i've put it onto a recessed sinar board and now to take some test shots with it. i don't have a centre filter so i want to see how it performs. and also to try it with a 6x9 back.

Bip

for note, when unscrewing the retaining ring if you don't have to correct shutter key/wrench you can often get a set of needle-nosed pliers or an open pair of scissors to work.