So the idiot in the macro dug up a conduit carrying the future cable TV line from my house about 120m to the road. It's good quality RG6 and the break is about 20m from the road where we expect to have service at some time in the next year or two.

The questions are:

What is the best way to fix the broken cable before sealing the conduit and laying it to rest again.
Will a fix of any kind stand the test of time (I'm thinking that corrosion due to moisture could become a problem)

On a related matter, someone from the same crew who hired the blind macro driver decided it was too difficult to pull the cable bundle (a different set to another building) and without thinking "Let's use that bigger size conduit over there" decided to cut two phone lines and two RG6 cables about half way into a 130m run and plans to install a junction box. As far as I'm concerned every connection is a potential loss in quality and future maintenance liability but of course they say I'm worrying about nothing. Am I?

So the idiot in the macro dug up a conduit carrying the future cable TV line from my house about 120m to the road.  It's good quality RG6 and the break is about 20m from the road where we expect to have service at some time in the next year or two.

The questions are:

    What is the best way to fix the broken cable before sealing the conduit and laying it to rest again.

    Will a fix of any kind stand the test of time (I'm thinking that corrosion due to moisture could become a problem)

On a related matter, someone from the same crew who hired the blind macro driver decided it was too difficult to pull the cable bundle (a different set to another building) and without thinking "Let's use that bigger size conduit over there" decided to cut two phone lines and two RG6 cables about half way into a 130m run and plans to install a junction box.  As far as I'm concerned every connection is a potential loss in quality and future maintenance liability but of course they say I'm worrying about nothing.  Am I?

Make sure it's not pudding jointed in the ducting, run new cables and seal the ducting. As for other buildings unless they are yours then just forget about it.

If I understand your question correctly. this is how I would repair such line brake

If there is enough slack on the line, simply cut the line, clean the ends, install male fittings on both ends,

-instructions here: http://www.wotsat.com/news/article/making-co-ax-connections/10316

-connect with a union

And insulate with electrical tape , a junction box, or shrink tube

edit for typo

Yes, that is the correct connector sirineou.

A couple of points:

Don't use ordinary electrical tape, use 'jointers tape', 3M make it and it's available here (I even got some in Tesco). You remove the backing and stretch it (3 x its length) before applying and when it shrinks back it amalgamates into a single waterproof lump (it's also called amalgamating tape). It's the same stuff that they use to joint overhead power cables so it must be good

Whatever you use don't make it inaccessible, the joint will fail at some point and if you can't get the joint down the tube you're stuffed.

To be honest, I would fix the conduit (properly) and pull a new cable.

They fixed it today with a crimped bnc joint and then the kind of tape that Crossy suggested. Just got to overcome my irritation with the incompetence and lack of forethought so I can turn my attention as to why the ceiling people hid 4 CCTV camera points and the electricians decided that my TV could do without its own LAN connection ("You've already got one going to the computer in this room"). Hmm.

They fixed it today with a crimped bnc joint and then the kind of tape that Crossy suggested. Just got to overcome my irritation with the incompetence and lack of forethought so I can turn my attention as to why the ceiling people hid 4 CCTV camera points and the electricians decided that my TV could do without its own LAN connection ("You've already got one going to the computer in this room"). Hmm.

Blame your foreman for the last two - you're paying him to understand the plan, and make sure mistakes like this don't happen.