Just an update on brothers issue.
Haven't seen him since the issue sorted itself out, but we regularly chat on the phone.

Back then he didn't really drive the TD5 every day, relied more on a small runabout for day to day, and TD5 on the weekend to get kids to cricket and stuff.

He told of an occasional(ie. very random) re occurrence of the power loss issue, no predictability in it coming back, but happened about three times, in about 20 starts.
Twice in a row, once just randomly .. no correlation to weather conditions, or anything like that. I'm thinking maybe in the rain, or humidity, or high heat .. but no consistency about it happening again.

Just happened or (usually) didn't.

As we were chatting it struck him that ever since the first service he and I did, the only thing that wasn't really thought about was the new injector loom we replaced as a pre-emptive choice on the oil in loom issue.
New injector loom, no idea on whether it was super low quality one or not .. assumed by price(about $60-ish from the UK) that it'd be a higher quality harness.
But since we did the FPR and had to change the return drain line, I did pull on the engine to ECU loom to get it out of the way, which must have stressed it in some way too.

Anyhow, I just told him to spray WD or whatever similar stuff he had with him on the plugs at the least, pull them and check pins too.
One pin was slightly bent, but still allowed plug from ECU to plug at the head to connect positively.

But in the two or so weeks now that he's done that, and driving it daily(run about got wrecked) .. he hasn't had a single stutter.

We still have plans to do those injector seals too(as he's got them now) again as a precaution, but so far in two weeks of every day driving(short and long trips) not a single return of the loss of power, stuttering or occasional starting issue he used to get.

So (fingers crossed) the diagnosis so far is a slightly dodgy injector harness to ECU loom plug.