RE: I've looked at the reference material and it appears to be US relevant rather than UK.
That's correct.
RE: The VAT fields don't appear to exist in the IIF file format any longer;
Yes. Unfortunately you can no longer specify VAT the way you used to. For some reason Intuit changed this almost a decade ago. You can specify tax-ability for each line item, and the total tax, and then QB will auto-select the default VAT code for the taxable and non taxable lines. It's rather inflexible and weak compared to the prior solution.
RE: The IIF process will default all SPL lines to the default tax code for the supplier which is not correct in the UK as we have several VAT rates that could be used by the same supplier;
Yes, that's how it works. No way to make it any smarter anymore (it sucks, basically, and we've heard as much from many of our customers after they upgraded.)
RE: The only way I could get an IIF Bill to import is to upload an out of balance Bill (Total Value and SPL value(s) without a balancing line which would be a bit of a challenge if there were a genuine out of balance occurring in an IIF file as almost all Bills wouldn't balance.
You're getting it... The definition of "in balance" is quite odd in these cases. I forget the details, but it looks like you have it.
