Last radiators on the heating 'loop', L/S fully open or not?

We have 2 large radiators (1.9kw and 2.2kw) which are the furthest from the boiler.
I have balanced all the others, some to 1/4 turn from off etc etc, but am unsure wether I should leave these last 2 rads fully open or balance to 12c or even more.
They are in an open plan room and are in the same room as the room stat (Nest).
No matter what I do I can't get the return and flow at the boiler, after it has been running for 15 mins or so, more than about 5C below the flow temp..
I even had an engineer out to check if the internal auto bypass was 'stuck'. He even checked if there was an external one anywhere and there wasn't.
I can run at 45c (rad mean is about 42c) in this below freezing weather but am wondering if I should (would) get better economy/performance with some thing like a 50/30 dt.
Thoughts please

I've had a couple long-run rads like that on my system and found that leaving them fully open usually works best, espacially if they're in the same space as the room stat. Trying to force a strict 12C drop on the very last radiators never really behaved for me anyway.
If your boiler is only seeing about a 5C delta after its been running a while, that normally points to high overall flow rather than anything wrong with the bypass. Some modern boilers just run that way unless you throttle quite a few rads.
You can try nudging the locksheild on those two rads a little, but I wouldn't chase perfect numbers. I've had better luck finding a comfortable flow temp and letting the system settle, rather then trying to hit a textbook 50/30.
If the house heats evenly and the room stat isn't overshoting, you're probably close enough already.

Thanks for the reply.
All the other rads are balanced so every rad is roughly the same temp. at about the same time.
What 'niggles' me though is, that no matter what flow temp. I set, be it 40, 50, 60 etc. then the return at the boiler eventually stabilises to about 5C below the flow temp.. This means that if I DID need a higher flow temp., say 70C, then the return would be ridiculous at about 65C even with rads balanced.
I have noticed that on these 2 last rads, even with l/s fully open, the delta is about 12C anyway. I suppose I could try having a bigger delta at these rads but I can't see it making any difference.
I'm currently running with all trvs at max as the flow temp through the rad seems to keep these at a reasonable temp in the other rooms (apart from one which obviously needs a bigger rad).

Johnytuono said:

then the return at the boiler eventually stabilises to about 5C.

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I assume when you say this you actually mean the return to the boiler stabilises at 5C BELOW the flow temperature? A return temp of 5C would be VERY low indeed!

ChrisJP said:

I assume when you say this you actually mean the return to the boiler stabilises at 5C BELOW the flow temperature? A return temp of 5C would be VERY low indeed!

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Oops, sorry yes, 5 C below the actual flow temperature.
Both posts now edited for clarification.