Dist/Time calculations
Whether you would regard an analysis done by Boeing as likely to be biased or not, the following document lays out its calculations and you can use it as an aide memoir to help you:
Mull of Kintyre -Analysis of Available Data
8-7D20-DS S-03 06, Enclosure 4
James Mitchell of the Boeing company
Section 3.2 “Aircraft Velocity Assessment” is not a bad effort.
The assumptions therein about wind speed are based upon the forecasts and, of course, what is known from measurements at the light house – the (significantly) increased wind speed on the Mull itself is what you would expect with an on-shore wind pushing against the slope and reference to wind farm studies should assure those who may be surprised at how much it would increase.
What is most relevant, I suggest, is that the gist of what the Boeing analysis is saying is that the a/c would have to have been doing the top end of cruise speed for the whole way until near the Mull (to have got where it did by when it did): it couldn't have averaged any slower up until that area – and, as this would have been up to the limit for passenger and crew comfort, it could hardly have gone any faster to make up for any slower periods – thus we can reasonably assume that they had not deviated from a straight track at the high average calculated for that long leg from NI to the Mull - that is, in simple terms, they had flown in a straight line at constant speed until close to the Mull – they could not have done anything else.
On the final leg from the position of way point change to the point of impact Mitchell uses the speed from the last steering calculation (over ground on the slopes) together with the strong wind there to deduce that the a/c had slowed down in terms of airspeed but that this had been masked by the increase in wind such that the ground speed had remained constant.
I add that the engine states (matched and not at the level to be expected for maintaining the high cruise speed over the preceding long passage) were consistent with an intended reduction in speed (say, coasting in) – the matched state (nor for that matter the power level) was not consistent with any kind of emergency situation and the last second manoeuvre apparently initiated was too sudden for the engines to have been disturbed from this matched state indicating the pilots being surprised at their proximity to the ground.