Yes, I was concerned about the long term stability of veg oil. After all, I wouldn't use 3 year old oil to fry my bacon!
Shell 'Dialla B' transformer oil is available in small quantities from Ebay. It's what Mikey 405 used in his.
For my own project, there will now be a break before I get a chance to do any more with my transformer. I think I will order some proper oil for it in the meantime.
Cheers
Nick