ROG:
TRAFFIC ANSWERS
I’m banned from their site. Tossers. 
tachograph:
I could be wrong but I think you’ll find that it’s illegal to leave a vehicle unattended with the engine running, unless the engine is needed for the use of auxiliary equipment.
Correct, with either the engine running or the parking brake not applied (under Con & Use). But the relevant term here is ‘leaving’. ‘Exiting’ the vehicle is not the same as ‘leaving’.
Exiting the vehicle to carry out other functions, such as, carrying out ancillary functions, is different to ‘leaving’ to nip into a newsagents to buy a paper. Whilst carrying out ancillary functions, you are still effectively ‘in attendance’ at the vehicle. 
A year or so ago, a car owner, washing his car on his drive, left the keys in and, whilst he was in the garage replenishing his bucket, or whatever, his car was taken. His insurance company initially refused to pay out but, an appeal to the ABI (I think) resulted in his claim being met on the basis that he had not ‘left’ the vehicle. He had merely turned his back whilst carrying out an associated function.
Whilst the decision is not binding on the Criminal Law, it serves as a form of guidance as to what should be considered in the realms of ‘reasonable’ conduct.
ROG:
IMO, the law makers ‘cocked up’ on the mobile phone issue - they should have said ‘no using any sort of communication equipment (emergencies exempt) unless the handbrake is fully applied’
I drive an automatic (car). I never use the handbrake. I use a ‘parking device’. 
And how, in evidence, is one to prove that a handbrake has not been ‘fully applied’. In Law, as in Science, “proving the negative” is always problematic.
Your argument also fails on the basis of Satnav. It is, after all ‘communication equipment’. In my opinion, the hysteria over mobile phones was just a knee jerk reaction to a limited problem. Whilst I won’t deny that there have been some horrendous accidents caused by the misuse of mobile phones, there have probably been a similar number, if not more, caused by the distraction of unwrapping sweets whilst driving. Or changing tapes, or C.D.’s. Or, dare I admit, smoking. 
Mobile phone use is measurable. Their use is traceable. Therefore it became a ‘measurable quantity’. What is not ‘measurable’ is the safety issues of the driver of a large vehicle being ‘talked into’ the location of difficult to find premises. In a strange town, or village, even with a mobile phone held to my ear, providing street by street, landmark by landmark, instructions to my destination, I’d be far safer than having to second guess as to which roads could appropriately accommodate the vehicle, and then find that I had to then reverse out. 