Taxes on 401k withdrawals?

Yep I think that's the way to go. Start to move sums over this year to a Roth and if need be take distributions from my Trad IRA in the years up to the first 5 year 'maturity'. I guess both the transfers to the Roth and any IRA withdrawals taking place in the same year will be counted as taxable income, so I need to add both together to get an overall picture of total tax payable / tax band, I also assume that starting to transfer funds from IRA to Roth doesn't generate any taxable event on the remaining money in the IRA - I can't imagine it would.

Then of course the next bit of fun will be Vanguard index funds ( which I have now) or Vanguard ETF index funds... ummm?

Yep I think that's the way to go. Start to move sums over this year to a Roth and if need be take distributions from my Trad IRA in the years up to the first 5 year 'maturity'. I guess both the transfers to the Roth and any IRA withdrawals taking place in the same year will be counted as taxable income, so I need to add both together to get an overall picture of total tax payable / tax band, I also assume that starting to transfer funds from IRA to Roth doesn't generate any taxable event on the remaining money in the IRA - I can't imagine it would.

Then of course the next bit of fun will be Vanguard index funds ( which I have now) or Vanguard ETF index funds... ummm?

The remaining money in the IRA stays tax deferred.

When it comes to ETFs or  Mutual Funds then it has no tax implications if inside the IRA or Roth. We have Vanguard after tax  index funds as well, so converted the funds to their ETF equivalents before moving back. No tax implications on the conversions but once back in the U.K. HMRC recognizes Vanguard ETFs as “reporting funds” so get the favorable, lower, tax treatment for dividends and capital gains.

Hello Durhamlad - last question , promise. Having thought about this overnight I was wondering if it is possible to make use of Foreign Tax credits, I have around 200K in FTC and an even lager amount in FTC AMT ( although I am never really sure what AMT is) as carried forward to 2020. Can any of these credits be utilized to reduce the one time US taxes that would occur with  the IRA to Roth IRA conversion?

Hello Durhamlad - last question , promise. Having thought about this overnight I was wondering if it is possible to make use of Foreign Tax credits, I have around 200K in FTC and an even lager amount in FTC AMT ( although I am never really sure what AMT is) as carried forward to 2020. Can any of these credits be utilized to reduce the one time US taxes that would occur with  the IRA to Roth IRA conversion?

I don't think so, but am not a tax expert by any means. The definition of a Foreign Tax Credit is that you have paid foreign taxes on income that you have also paid domestic taxes on.  In the case of IRA to Roth conversions tax has only been paid to the IRS, not to HMRC. In other words no foreign taxes have been paid on a Roth conversion so there is no double taxation that can utilize FTCs.

I also have FTCs carried forward and I honestly don't know if I'll ever be able to use them as the taxes I pay to HMRC are always more than what I pay to the IRS and I can't see that ever changing to where I pay more taxes to Uncle Sam than QE II. (In that case I can use accumulated tax credits)

AMT exists to limit the number of deductions that rich taxpayers can take, compelling them to pay more taxes. I've never been subject AMT even at the peak of my and my wife's earning capacity. Retired now I we will never be subject to AMT unless they greatly simplify the tax system by eliminating many tax deductions and make everyone pay AMT.