This is only an update, have no new questions.

Took another look at average weather conditions, decided to bring one short sleeve jersey and one long sleeve jersey instead of two long sleeve.  And made some other changes in my rain gear plans.  Leaving the sandals at home.  Bringing hiking shoes with waterproof gaiters, I hope if I wear the hiking shoes in the rain without rain pants that the water dripping down my skin does not get inside the gaiters, which it probably will do to saturate the socks and hiking shoes.  Tentatively bringing two long sleeve shirts, one pair long pants for off bike wear.  Besides the two jerseys, bringing two pair of bike pants that can be converted to bike shorts.  That said, when I finally pack and see how much weight and volume my stuff consumes, the list should get shorter later, it always does.

My helmet mounted mirror has a small threaded clamp that clamps onto my visor on the left side.  I discovered that I could do a bit of surgery on the visor for my helmet and that allowed me to put my mirror on the right side of my helmet.  That should work great.  I will bring the eye glasses mounted mirror as a backup, but I anticipate no problems with my helmet mounted mirror on the right side.  My left eye has better vision than my right eye, but there is nothing I can do about that.

Made a hostel reservation for Perth late in my trip for one night before I arrive in Edinburgh at the end of my trip.  I have hostel reservations for the first five nights and the last four nights in UK.  I now have 34 consecutive days with no reservations, but planning on camping most of that time, especially when in England, so I am not concerned about the lack of reservations.  I am sure I will have some zero distance days, typically weather related so I do not want to make more reservations at this time.

I have lots of tracks and waypoints saved in my GPS for flexibility in routing and where to stay at night when in Scotland.  I really do not have a schedule, i will just do what seems logical when I can see the shorter term forecasts, while checking the longer range ones to make sure I can finish on time.

Am mostly packed.  As usual I have too much stuff packed.  But in the past I have regretted some of the last minute shedding of packed items, so properly lightening the load is always difficult.

I want to thank everyone that has made some suggestions.  In some cases I did not follow them, but I considered every suggestion.

I leave on June 2, sleep on the plane, arrive on June 3 in EDI.  Long range forecast suggests I will need rain gear on the day I arrive and for several days after.  But forecasts over a week out can be quite inaccurate

Today it looks like London temp is 35 (C), same with tomorrow.  At that high an altitude, I can't figure out what is going on, but I hope things are closer to normal when I am there, if I wanted to go somewhere that hot, I can think of a lot of other places to go.  If things look like this just before I leave home in a week, I may bring a pair of shorts and a short sleeve shirt.  But I really hate to bring anything that I might only use once or twice for a month and a half long trip.

I have no questions at this time, but if anyone is aware of anything that may disrupt my plans, please let me know.  I am aware that there may be a jet fuel shortage in mid July.  I recently read that some of the European refineries are able to produce more jet fuel than previously anticipated, thus the shortage should not be as bad as initially predicted.

Disappointing on the fittings, if that's so. Although realistically it would be a real-world problem only for your grandchildren, unless you're planning a few round-the-world trips. A paint downgrade would be disappointing too.

I'm still really enjoying my Mercury Mk3 650B. Perfect for my use envelope of day rides and light touring, tar and gravel.

Hard to know whether the 40 will be as good, especially as I doubt many Mk3 owners will trade for a 40. Probably it's not quite as good, but that's still very good.

The Mk3 is a great frame for a Rohloff because the rear dropout is designed to prevent the hub outer from rotating without the need for a torque arm, and the cable runs are  Rohloff specific and well thought out. Albeit you still need to protect the frame from the cable outers, in places.

One of my vintage VDO computers from over a decade ago died this past year, too much use in the rain.

I gave up looking for another computer like it with a cadence feature.  So I bought a couple cadence sensors to add to my crank arms, will use the GPS unit for that on my Nomad Mk II and my road bike instead. 

Downside is that sometimes for an exercise ride near home, I do not bother bringing the GPS along since I know where I am going.  Which means now no cadence on those two bikes when I do that.  I can live with that.

Matt,

Per the instructions, you'll need to press on the inner seal cap just above where it is marked "TOP". That should elevate the opposite side, allowing you to pick it off.

See attached, magnified screenshot.

You'll have to reinitialize after. See...
https://www.cateye.com/intl/support/manual/data/doc/CC-RD300W_ENG_v10.pdf

Best of luck, Dan.
[EDIT: I think your inner seal cap may be mis-oriented so the TOP marking is not at the top in relation to the unit body. This may make it more difficult to "press-and-pick off".]

An inner cover and an outer cover?  Inner cover, I would expect it to have a slot if it is supposed to be rotated, or an arm that sticks out to one side.

If you have the model number, perhaps the instructions are out on the internet?

Thanks Mick
The picture shows the outer cover removed. It's below the main body.
The cover for the battery won't come off. There is no groove for a screwdriver blade. And nothing to grip the cover with.
Very frustrating.
I don't have the model number to hand but do have the printed instructions that came with the unit.
The instructions don't make it clear how to remove the battery.

Thanks for your comments