Stephen Haley wrote:

I don't understand clearly what is the problem at Dunstable. What is the
length of runway ahead of you when you reach 150m?

Simple answer is not really enough when launching in flat calm conditions
which aggravate matters in 2 ways
1) Lack of wind reduces the overall height gained by upto 30% which means
you will be substantially further down the field @ 150ft.
2) Lack of headwind means that the landing will be longer.
10kts of wind changes everything and produces a reasonable overlap.

The ordinance survey shows the field at about 1k in length

I was talking of 150m, not 150ft. If your 1k length is also in feet,
this is pretty short (~ 300m), i.e. the winch is probably far beyond the
landable area, which is unusual. If it is 1km, even if you had a very
poor climb and you are at mid field (500m) at 150m, 500m seems highly
sufficient in any condition of wind for a landing straight ahead.