Though not common, any spool of line can come with a joining knot from the manufacturer. They use simple knots with little or no strength for this. These knots need to be replaced at once with a Blood Knot or something like a Double Grinner. Used diligently with due care, with the right kite, the right knots, and the right wind, good quality line should last many, many flights.
When a line breaks unexpectedly at what seems to be less than its rated test strength, remember that lab test conditions are different from real life in the field. In the lab a short, standard length of line is tested by applying a slow, steady pull. It is not tested with shock loads. A shock load, such as a quick, strong jerk or tug from a thermal or a gust of wind, can cause line to part.
Other reasons for lines breaking in flight are strong wind and gusts, along with abrasion damage, general wear and tear, bad knots, kinks, nicks, age, sharp clips, snags, crossing another line, or contact with anything with rough or sharp edges, and being put away wet and/or dirty. UV damage may come into play as line ages. Nylon line gets brittle after about 20 years, but this hasn't shown up (yet) in our Dacron and Kevlar lines in well over 15 years.
I have been categorically assured that UK-Hooks' X-Tension and Blue Marlin Dyneema lines, like TUF-Line, can indeed be knotted without the need for sleeving using similar knots to those used for TUF-Line XP.
We have been advised to avoid cheap lines from the Far East. We started our own trial with the same breaking strain we'd normally use, initially 300yds of 80lb TUF-Line XP.
Strong as they are in tension, rumors circulating about 15 years ago had it that these lines can cut through themslves at knots and so forth. However, that weakness in shear applies more to Kevlar than to its up-to-date replacements. These new lines appear to be very strong in shear - it is impossible to just snip a bit off even with sharp normal scissors, and they require a firmly held loop. The fact that they will cut through leather gloves is not just a rumor: leather, skin and tendons are no match for it should it slip from one's grip while a kite is pulling hard. Kevlar gloves have been recommended, though mine are a loose fit, so I currently still use leather sometimes (but don't take that as a recommendation).
My main complaint about high-tech thin lines for kite flying had always been that they were meant to be sleeved (fed through as woven sheath) at knots, where, being weak in shear, they would cut through themselves at much less than maximum pull. If it ever became necessary to do a quick line repair in the field, sleeving would have been tedious, but now it looks as if there's no need to worry.
UK Import Duty
As of April 2012. If the value of the parcel is £15 or over, there are fees including duty (I think it's 9%), an £8 administration charge and 20% VAT, no matter how little the value is in excess of £15, and it's not possible to argue the exchange rates.