I'd just take it on the road trip, and then check it when you get back. If it's no different, then just leave it until it becomes a problem.

If the bearing is going, I'd expect it to start whining, and there to be some debris on the drain plug... heck, even my brand new rear diff that was fitted 5 yrs ago had a bit of paste on the drain plug..

I think the £350 would be better spent on fuel at the moment.

2000 miles of fuel. Now that really would be a road trip!
I'll drop the prop tomorrow, just in case it's as simple as the lock nut working loose. Re-torque and see if the play goes away. Preload on the pinion is set with shims so shouldn't make anything any worse.

My original rear diff had about 3mm of in and out play on the input, the best part of a quarter turn slop and a healthy clunk whenever I went from forward to reverse. Did whine a bit like a London bus but it didn't get any worse in about 50,000 miles. I only replaced it because I thought I should.

Strange that the pinion oil seal isn't leaking, as the flange will be orbiting in the seal, rather than rotating.

I think you're seeing a problem where there isn't one, OB

Oh there's definitely a problem Shep. It's just the scale of the problem.
From an engineering point of view there should be effectively zero lateral movement in the pinion flange. Lateral movement will transfer through the driveline- the end of the prop will effectively orbit rather than rotating. This will cause rotational vibration which will impact the next "fixed" point in the system- the transfer box output bearing, as well as killing the UJ's.
That's the extreme consequence. mine isn't there- yet.
The heavy mass damper on the diff will tend to mask the vibration (that's kind of what it's there for).
What engineering analysis can't tell me is how long the failing bearing will last.
That's the quandary. It needs fixing. The question is when?!

Refurb diff and new prop UJs now fitted. Took it out for an instrumented (on Nano) run today, mainly just to check for high speed vibrations, whines, wheel imbalance, that kind of thing. All good which is nice.
What's not so good is the Nano suddenly started beeping in a panicky kind of way -coolant temperature warning. Needle on temp gauge steady just below centre, like it always is.
Idle along for a bit to take heat load off. Warning stopped. Read data and sure enough water temp was running between 95 and 100.
Fan's good, water seems to be circulating happily judging by the jet squirting back into tank. Check rad (new RR rad and water pump fitted 80000 miles ago), bottom cold as was pipe to thermostat. Time for a new thermostat I think. Hooray, I get to drain the cooling system again!

Oh joy

At least you know what it is and it's not an overly expensive fix :)

You've gone awfully quiet Mark, haven't you got those new heads on yet? What's the state of things at the moment?

I'm still around- I've just moved away from my "stream of consciousness" postings where I was posting too much inconsequential cr@p relating to my builds and thought processes! Just getting on with doing it...
New heads are on. Castings must have been in their first iteration, or valves beautifully topped as didn't need any shims on pedestals, so I've got a large assortment of shims to hand for the next head job.
Lower inlet manifold was slightly damaged as whoever fitted it previously had not clamped the rear end seal correctly, rather they'd trapped it partially up the manifold, distorting the clamp and messing up clamping face, but that's sorted- just waiting for a new clamp to turn up.
Other than that, new QH water pump, new thermostat, new temp sender, bunch of new hoses and away she goes, again!

Patience is back on the road, just in time to take the other one back into the workshop as the water in oil problem isn't going away.
A few teething problems, some anomalous readings between gauge temp and ECM temp (covered in another thread) and a very slight leak from RHS exhaust manifold to downpipe- only audible because the engine is now so quiet and smooth,
Of course, now the engine is so quiet, the loudest thing is a gearbox whine. You couldn't hear it if there was an aftermarket exhaust on the car, it's that small a whine, but it's there.
Just driving it and enjoying it at the moment. Only pending treatment is an appointment to get the headlights properly aligned next week. My chalk line and tape measure setup was pretty close but those LED lights are so white and bright that I want to make sure they're legal and to MOT standards.
After that- on with the paintwork...

What was the issue with the coolant sensor in the end?

Never got to the bottom of it, or rather haven't yet!
What I'm lacking is data for IR thermometer for a normally presenting ECM (Nano) coolant temperature (ie readings at top, bottom rad hoses and body of temperature sender with engine idling fully up to temp and stat open, compared with Nano readings. Whilst I can measure mine, I don't have anything to compare them with.
Cables and connectors to sender are good. Only other thing that it might be is a hotter thermostat. I fitted a new one when I did the head work, but did test it in a pan of boiling water first to check that it opened/ closed at the correct temps. and it did.
Bit baffled really and still slightly anxious about cooking the rebuilt engine, although telling myself if I didn't have a Nano things would appear perfectly normal.