I suspect that my central heating system may not be perfectly balanced, as
the radiators that are furthest away from the boiler and pump are rather
under-powered (and often don't come on at all).

First, am I right in thinking that this is likely to be a balancing problem?
We've recently had all the radiators cleaned out and de-sludged.

I've had a look at what's involved in balancing, and I see that there is a
recommended temperature drop of about 11 degrees C between the two ends of
the radiator. How critical is that? If I am going to miss that, is it more
serious if I miss it in one direction or the other?

As far as I can tell, if I turn the lockshield valve too far towards the
closed position (that would give a bigger temperature drop, right?) in some
of the radiators nearest to the boiler, that's not going to do any harm
provided that they get hot enough themselves. Is that correct?

Many thanks for any hints and tips.

Adam