Thank you so much for your reply to my dilemma... I take on board your information and guidance. When I refered to the Phase One Lifgtphase method I was recalling back to early MF backs where an IR filter was attached in front of the lens as there wasn't one present on the back. I remember using one which I'd hired a few years back and I attached the filter very simply with a bit of blue-tack! Not very scientific, but it worked.
I was wondering in theory if I removed the IR filter off my back and then used an external IR filter in front of the lens if this would achieve the same purpose or if there would maybe then be a colour cast with the difference in glass etc...
Unfortunately the scratch was not disclosed on the listing, it was cheap though!
Thank you, Craig