Paul Weaver wrote:
I'm a little unsure about the law on box junctions. As I understand
it you can enter a box junction when turning right if the road you
are turning into is clear and when oncoming traffic stops you can
proceed.

Agreed.

What happenes if this road then fills up as you are waiting (by
oncoming cars turning left)?

The law doesn't seem to cover that circumstance, and while it might seem
unfair, the law as written seems to say that it's an offence to remain
on the yellow box after oncoming traffic has cleared. The text is at
http://www.hmso.gov.uk/si/si2002/20023113.htm . (Search for "box
junction".)

One other question, are u-turns automatically enforced, or is it
leanient (e.g. no fine for uturns on an empty road at 3AM)

AFAIK, if you're not in a one-way street then U-turns are permitted
provided they are not specifically banned by signs.
--
Richard J.
(to e-mail me, swap uk and yon in address)