In reply to deepsoup:
My knowledge of this is a bit rusty but,
It is a defence "to prove that at the time he is alleged to have committed the offence the circumstances were such that there was no likelihood of his driving the vehicle whilst the proportion of alcohol in his breath, blood or urine remained likely to exceed the prescribed limit ."
Roughly speaking in order to "prove" you are not likely to drive in those circumstances (as a result of the Human Rights Act and a case of DPP vs Sheldrake) it has been interpreted to mean "present an arguable case".
The best way of doing that would include being to be in the car without the ignition keys.
Failing that (and this is my opinion rather than legal advice
- I would suggest if you are tucked up in your sleeping bag, with all your climbing kit and guidebooks, with a definite plan for the next days activities (that can be corroborated by your climbing partner) then I think you would have an "arguable case".
Bear in mind that if you have had 15 pints, left the pub at 2am and are planning a 6am start - then you "could" run the risk of being hit due to the level of alcohol in your blood when you are planning to drive.
However, regardless of all the above, it would be entirely lawful to breathalyse you, arrest if you blew over or failed to provide and take you into custody to carry out an intoxilyser test and gather evidence by interview etc.
Again not advice - but I would suggest that you are better off blowing into the roadside breath kit if you are asked - fail to provide = arrest.
Similarly if you were taken into custody then I would suggest blowing into the intoxilyser as fail to provide will then result in a charge.
Common sense would suggest that it might be wise to be friendly and polite
Realistically I can't see why cop would waste their time going through all that rigmarole in the middle of the countryside with someone who is clearly tucked up in a sleeping bag with axes, though they might want to prevent you stealing some good lines if they think conditions are good, and they can't get on the hill the next day
NB - All this is English rather than Scottish law (which may be the same - but I haven't checked)