In case it is of interest - finally got things ticketed - took a while and some pretty big frustrations with BA - but like always - if you find the right agent then things can be done right and quickly.
The DONE4 ended up being 4193 GBP + 2888 GBP in Taxes - so just over 7K GBP for 14 segments (+2 surface) in Business and around 38.5K miles to be flown