Hi David,
Great to hear that you have achieved results on items i) and ii) - That's Brilliant ! - and my respect for 'owning up' to the 'operator error' with the RADAR Screen settings - that always makes things a lot more straightforward (and after all we ALL make mistakes).
Also OK on checking the microSD card contacts and 'fit' of the card and that nothing undue was found.
As I have stated above, from my analysis of your track logs, you haven't been experiencing GPS dropouts, but shutdowns of the Rosetta. The only way to confirm this for sure is by running the unit in as near to the same 'real life' conditions as you were before, for several prolonged periods (I'd suggest at least a couple of hours at a time in view of the fact that we know it ran previously all the way from Sandown to the Manchester LLC). If you aren't able to fly, you can certainly 'soak test' the unit on the ground without the Transponder, though there is no 'guarantee' that it will perform the same as it did in flight. In particular you need to make sure that the GPS has a good clear view of the sky or this will obviously skew the results. If you then download and check the track files, you will fairly quickly recognise the logging pattern and will easily be able to check the timestamps at the start and end of each log and compare these with the times that you ran the unit for. If the log stops short of the end of the test period, then the unit must have shut down, in which case you need to try a new software install - preferably on a new microSD card to rule out a card fault. That's certainly what I would do myself.
Let me know if you need any help.
Best Regards
Peter