I know that this might become an emotive issue but what is the perceived wisdom here on running in new pistons and barrels?

Some say nail it from day one to glaze the barrels and make the rings seal immediately and others believe in driving like a saint for a few hundred miles at varying speeds?

Nels.
there is a happy medium between the choice of two extremes you've given.
This advice has worked well for me when new pistons and barrels, or just rings, have been fitted.

From. http://www.citroentuning.de/doc/How_to_break_in.pdf

"Now you must break in your engine on a road or highway:
This can easily be done following this procedure.
Look for a road that allow you to do the following: For fast and safe break in of
pistons, rings and cylinder we recommend to accelerate your car with floored
accelerator from 40 Km/hr in the greatest gear (Fourth or fifth gear) to maximum
speed the engine will reach. Never force the engine to rev up more than it´s
willing to do ! Reaching the speed of around 90/100 Km/H remove your foot and let
the car (without disengaging the gear or clutch) decelerate and slow down to
approx. 40 Km/hr. Then reaccelerate with flooring your accelerator to repeat this.
Do that 15 times and drive 50 Km “normally”.
When accomplished your engine is really break in to 80%. The last 20% of engine
break in is done during daily driving but don’t force your engine to RPM levels that
it will not reach or too high RPM or to high load with a long gear during hills
climbing"


ken

you want to load up the rings on the cylinder wall which involves putting a bit of strain on the engine as above.

you dont want to" glaze" the bore as that leads to blowby and oil problems - the best way to glaze the bores is to pussyfoot about and let it idle for ages when cold

while ive put new rings in an engine and sent it off to do the 24hrs on many occasions its not the ideal way to run it in but is closer to the recommendation above than trundling about like a granny

being gentle withe the engine was for all new engines where you had to give consideration to the bearings as well.

I had a nouveau exchange engine in mine back in 1994. I ran it in gently like ken suggests during my 50 mile each way commutes to work along the A55. After a week of this it had an oil and filter change. It ran beautifully after this and was the quietest 2cv you could imagine. It also pulled 80mph on the satnav, and would regularly keep the speedometer off the clock all the way down the M40 (apart from the stokenchurch cutting ).