An update to say I called back today to check on the holding of the return seats and was told the domestic miles seats had been released and it was all rebooked so a big ^ to BA for that. Its a little tip that may come in useful in the future.
A further ^ to the lovely girl I spoke to in NCL who waived the service fee - apparently as the origin was changing from LHR to NCL there should have been a back office fee but after I explained I hadn't been given the option to request the release of miles seats when I originally booked she didn't charge me the £60 so all in all a very good result.