It is naive to think there are only 2 types of bike thief - the opportunistic chav who nicks a bike for a laugh and probably ends up dumping it, or the tooled up hardcore crims who drive round wielding angle grinders and boltcutters and threatening anyone who tries to stop them.
There ARE people who drive around in vans looking for bikes to steal on an industrial scale. They ARE opportunistic. They know that the simplest/quickest way to nick bikes is not to make themselves a focus of attention by using tools, and they WILL look for unsecured bikes REGARDLESS of the reason they are unsecured. These are organised criminals who specifically target motorway services, laybys, picnic areas, bike parks, etc for this very purpose.
Hopefully your expensive bikes do not fall foul of such people. Thanks for your input and for raising the issue of the DL3 which I will avoid based on your advice.